Speed change system for work vehicle

ABSTRACT

A speed change system for work vehicle having a continuously variable transmission device selects a first change gear ratio which is set larger than a smallest change gear ratio, when an engine rotational speed is a first set rotational speed which is set equal or close to an idling rotational speed of an engine; retains the change gear ratio of the continuously variable transmission device at the smallest change gear ratio, when the engine rotational speed is equal to or above a second set rotational speed set on a high-speed side relative to the first set rotational speed; and makes the change gear ratio of the continuously variable transmission device larger between the first change gear ratio and the smallest change gear ratio, as the engine rotational speed at that moment becomes lower, when the engine rotational speed is between the first and second set rotational speeds.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.13/889,733, filed on May 18, 2013, which is a continuation of U.S.application Ser. No. 12/968,639, filed on Dec. 15, 2010, now U.S. Pat.No. 8,447,479, which are hereby incorporated by reference in theirentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a speed change system for a workvehicle including: a pump swash plate operation mechanism configured tooperate a swash plate of a variable capacity pump for continuouslyvariable transmission device; a motor swash plate operation mechanismconfigured to operate a swash plate of a variable capacity motor for thecontinuously variable transmission device; a swash plate angle controlunit for controlling the pump swash plate operation mechanism and themotor swash plate operation mechanism; and an operation positiondetector configured to detect an operation position of a speed changeoperation tool. A swash plate angle of the variable capacity pump isnon-stepwise changed, and a swash plate angle of the variable capacitymotor is stepwise changed.

2. Description of the Related Art

For such a speed change system for a work vehicle, Japanese UnexaminedPatent Application Publication No. 6-17928 describes a system in which:by changing a swash plate angle of a hydraulic pump (variable capacitypump) of a hydrostatic continuously variable transmission device throughthe operation of a shift lever (speed change operation tool), a changegear ratio of the hydrostatic continuously variable transmission deviceis non-stepwise altered, and based on detection information from apressure detecting unit for detecting a circuit pressure of a hydrauliccircuit that communicates with the hydraulic pump and a hydraulic motor(variable capacity motor) of the hydrostatic continuously variabletransmission device, a hydraulic motor is switched to a low-speed statewhen the circuit pressure of the hydraulic circuit is a predeterminedpressure or more, and to a high-speed state when the circuit pressure isbelow the predetermined pressure. In this system, though the hydrostaticcontinuously variable transmission device are provided with the variablecapacity pump and the variable capacity motor, the variable capacitymotor is normally kept in a high-speed state, and cannot be changed to alow-speed state by a manual operation. Accordingly, even when the speedchange operation tool is operated to an operation position on alow-speed side for performing a low-speed traveling of the work vehicle,the variable capacity motor remains in the high-speed state with a lowtorque. As a result, it becomes difficult for the work vehicle tosmoothly travel in a large load situation, such as hill-climbingtraveling and a working state in which a working device is connected. Inaddition, in a low-speed region in which the change gear ratio of thehydrostatic continuously variable transmission device is made large byoperating the speed change operation tool to the low-speed side, anangle on a low-speed side exhibiting low hydraulic pressure transmissionefficiency is more frequently used as the swash plate angle of thevariable capacity pump, and thus a vehicle speed is unlikely to bestabilized during the low-speed traveling.

Accordingly, it may be proposed to provide a special operation tool thatallows a manual switching operation of the variable capacity motor.However in this case, a number of operation tools mounted in theoperation part should be increased, and it becomes necessary to performa speed change operation by the speed change operation tool as well asan operation of the operation tool specialized for shifting the swashplate angle of the variable capacity motor, as speed change operationrelative to the hydrostatic continuously variable transmission device.Therefore, there is a room for improvement in operability.

As a vehicle speed control system for the work vehicle, JapaneseUnexamined Patent Application Publication No. 7-23610 (see paragraph[0012] and FIG. 1) describes the system including: a hydraulic cylinderfor speed change configured to operate a trunnion axis of thehydrostatic continuously variable transmission device (corresponding tochange gear ratio altering unit); a controller configured to control anactuation of the hydraulic cylinder for speed change (corresponding tovehicle speed control unit); a potentiometer type speed setter fordetecting an operation amount of the shift lever; and a potentiometertype feedback sensor configured to detect an actual turning amount ofthe trunnion axis, wherein the controller controls the actuation of thehydraulic cylinder so that a detection value of the feedback sensormatches a set value by the speed setter. In the work vehicle, it isoften the case that the change gear ratio of the continuously variabletransmission device is set to a small change gear ratio (high-speedside) for the purpose of, for example, enhancing work efficiency. Whenthe work vehicle is started in such a setting condition, a large loaddue to the small change gear ratio will be applied to the engine from astage with a low engine rotational speed, and therefore, engine stalldue to overload is likely to be generated.

Accordingly, in order to prevent generation of engine stall, anaccelerator operation should be carefully performed. Especially in acase where the work vehicle is started in a situation where a load islarge, such as in ascending a slope or in a working state with theworking device connected, there is a notable tendency that engine stalloccurs, and the accelerator operation should be further carefullyperformed. Therefore, there is a room for improvement in operability.

As a speed change control system for the work vehicle, JapaneseUnexamined Patent Application Publication No. 7-23610 (see paragraph[0012], FIG. 1) describes the system including: the hydraulic cylinderfor speed change configured to operate the trunnion axis of thehydrostatic continuously variable transmission device (corresponding tochange gear ratio altering unit); the controller configured to controlthe actuation of the hydraulic cylinder for speed change (correspondingto vehicle speed control unit); the potentiometer type speed setter fordetecting the operation amount of the shift lever; and the potentiometertype feedback sensor configured to detect the actual turning amount ofthe trunnion axis, wherein the controller controls the actuation of thehydraulic cylinder so that the detection value of the feedback sensormatches the set value by the speed setter. In this system, by a controlactuation of the vehicle speed control unit, the actual turning amountof the trunnion axis of the hydrostatic continuously variabletransmission device can be made correspond to the operation amount ofthe shift lever. However, it has been known in the hydrostaticcontinuously variable transmission device that, as the load becomeslarger, a loss of a hydraulic pressure due to oil leakage becomeslarger, resulting in reduction of power transmission efficiency.Therefore, even when the engine rotational speed is constant, and at thesame time the actual turning amount of the trunnion axis of thehydrostatic continuously variable transmission device matches theoperation amount of the shift lever maintained at a predeterminedoperation position, in a case where the load fluctuates duringtraveling, the power transmission efficiency in the hydrostaticcontinuously variable transmission device is then fluctuated, and thusthe change gear ratio of the hydrostatic continuously variabletransmission device becomes unstable and the vehicle speed changes to alarge extent.

Accordingly, with respect to the work vehicle, such as a tractor, inwhich power from an engine whose speed is changed by the hydrostaticcontinuously variable transmission device is used for traveling andpower from the engine whose speed is not changed by the hydrostaticcontinuously variable transmission device is used for work, if theconfiguration described in Japanese Unexamined Patent ApplicationPublication No. 7-23610 is introduced, in the case where work, such asrotary tillage work, is performed in which the vehicle speed and anactuation speed of the working device are desired to be retained in arelationship in which they are proportional to the engine rotationalspeed, this relationship cannot be retained due to a large vehicle speedchange caused by the fluctuation of the power transmission efficiency inthe hydrostatic continuously variable transmission device, and thus itbecomes difficult to make a working trail of the working device uniformwith high accuracy. In other words, there is a room for improvement inenhancing work accuracy of rotary tillage work or the like.

It has been proposed that, in the work vehicle having the continuouslyvariable transmission device, the change gear ratio of the continuouslyvariable transmission device is divided into multiple speed changesteps, and when the change gear ratio of the continuously variabletransmission device is stepwise changed among the divided speed changesteps, an integer value corresponding to the number of the divided speedchange steps, for example, in a case where the change gear ratio of thecontinuously variable transmission device is divided into seven steps, afigure from “1”-“7” is displayed as the speed change step of thecontinuously variable transmission device, and when the change gearratio of the continuously variable transmission device is changed in acontinuous manner, unlike the stepwise change in which the integer aloneis displayed, a decimal indication of a value from 1 to 7 is displayedso as to show the continuous change of the change gear ratio of thecontinuously variable transmission device. Regarding this technique, forexample, Japanese Unexamined Patent Application Publication No.2006-70943 can be referred to. However, it describes a technique inwhich the change gear ratio of the continuously variable transmissiondevice is decimally expressed merely based on the integer valuecorresponding to the divided speed change step, and therefore, such atechnique does not have enough meaning to introduce to the work vehicle,and it requires further elaboration and creation to introduce such atechnique.

SUMMARY OF THE INVENTION

Therefore, it is desirable to provide a speed change system for a workvehicle having more excellent performance, without reducing operability.

Such a speed change system for a work vehicle will be listed below.

(1) A speed change system for a work vehicle having a continuouslyvariable transmission device including: a pump swash plate operationmechanism configured to operate a swash plate of a variable capacitypump for the continuously variable transmission device; a motor swashplate operation mechanism configured to operate a swash plate of avariable capacity motor for the continuously variable transmissiondevice; a swash plate angle control unit configured to non-stepwiseadjust a swash plate angle of the variable capacity pump through thepump swash plate operation mechanism and stepwise adjust a swash plateangle of the variable capacity motor through the motor swash plateoperation mechanism; and an operation position detector configured todetect an operation position of a speed change operation tool; whereinthe swash plate angle control unit sets the swash plate angle of thevariable capacity pump and the swash plate angle of the variablecapacity motor based on an output from the operation position detector,and controls the pump swash plate operation mechanism and the motorswash plate operation mechanism so that the swash plate angle of thevariable capacity pump and the swash plate angle of the variablecapacity motor become respective set swash plate angles, and when theswash plate angle of the variable capacity motor is changed based on theoutput from the operation position detector, the swash plate anglecontrol unit controls the pump swash plate operation mechanism in such amanner that the swash plate angle of the variable capacity pump ischanged with a control amount which is set so as to compensate an amountof change in a change gear ratio of the continuously variabletransmission device caused along with the change of the swash plateangle of the variable capacity motor.

According to this system, by operating the speed change operation tool,not only the swash plate angle of the variable capacity pump but alsothe swash plate angle of the variable capacity motor can be changed.Accordingly, for example in a large load situation, by operating thespeed change operation tool to change the swash plate angle of thevariable capacity motor to an angle on a low-speed side, a high torquecan be obtained, and thus even in the large load situation, the workvehicle can smoothly travel. In addition, during the low-speedtraveling, by operating the speed change operation tool to change theswash plate angle of the variable capacity motor to the angle on thelow-speed side, an angle on the high-speed side exhibiting highhydraulic pressure transmission efficiency can be more frequently usedas the swash plate angle of the variable capacity pump, and thus thevehicle speed during the low-speed traveling can be stabilized, tothereby facilitate work at a low-speed.

Then, the special operation tool for shifting the swash plate angle ofthe variable capacity motor becomes unnecessary and therefore, itbecomes unnecessary to secure a space for mounting such a newlyintroduced operation tool in the operation part, and lowering in theoperability of the continuously variable transmission device can beprevented, which may otherwise be caused by introducing the specialoperation tool for shifting the swash plate angle of the variablecapacity motor.

Furthermore, when the swash plate angle of the variable capacity motoris shifted from the angle on the low-speed side to the angle on thehigh-speed side, along with the shifting, the swash plate angle of thevariable capacity pump is changed to an angle on a low-speed side withan appropriate operation amount, to thereby compensate (offset) anamount in the change gear ratio increased along with the shifting of theswash plate angle of the variable capacity motor. To the contrary, whenthe swash plate angle of the variable capacity motor is shifted from theangle on the high-speed side to the angle on the low-speed side, alongwith the shifting, the swash plate angle of the variable capacity pumpis changed to the angle on the high-speed side with an appropriateoperation amount, to thereby compensate (offset) an amount in the changegear ratio reduced along with the shifting of the swash plate angle ofthe variable capacity motor.

With this configuration, while the swash plate angle of the variablecapacity pump as well as the swash plate angle of the variable capacitymotor can be changed by operating the speed change operation tool, thechange gear ratio of the continuously variable transmission device canbe made constant, regardless of the shifting of the swash plate angle ofthe variable capacity motor. As a result, while the swash plate angle ofthe variable capacity motor is configured to be stepwise shifted, thechange gear ratio of the continuously variable transmission device canbe altered by the speed change operation tool, smoothly without steps.

(2) A speed change system for a work vehicle having a continuouslyvariable transmission device including: an engine rotational speeddetector configured to detect an engine rotational speed; and a vehiclespeed control unit configured to control a change gear ratio of thecontinuously variable transmission device; wherein the vehicle speedcontrol unit controls the change gear ratio with a process including:(a) when the engine rotational speed is a first set rotational speedwhich is set to a rotational speed equal or close to an idlingrotational speed of an engine, selecting a first change gear ratio whichis set to a change gear ratio larger than a smallest change gear ratioas the change gear ratio of the continuously variable transmissiondevice; (b) when the engine rotational speed is a rotational speed whichis equal to or above a second set rotational speed which is set on ahigh-speed side relative to the first set rotational speed, retainingthe change gear ratio of the continuously variable transmission deviceat the smallest change gear ratio; and (c) when the engine rotationalspeed is a rotational speed between the first set rotational speed andthe second set rotational speed, making the change gear ratio of thecontinuously variable transmission device larger between the firstchange gear ratio and the smallest change gear ratio, as the enginerotational speed at that moment becomes low.

According to this system, in a case where the engine rotational speed isincreased from the idling rotational speed by an accelerator operationand the work vehicle is started, as the engine rotational speed becomesa lower rotational speed close to the first set rotational speed, thechange gear ratio of the continuously variable transmission devicebecomes a larger change gear ratio on a low-speed side, and thus a loadon the engine is reduced. Therefore, as compared with the case where thework vehicle is started in a state in which the change gear ratio of thecontinuously variable transmission device is retained at a small changegear ratio on the high-speed side exhibiting a large load, withoutplacing any burden on the driver, a decrease in the engine rotationalspeed and generation of engine stall caused by overload upon startingcan be effectively suppressed, and the starting of the work vehicle canbe smoothly performed.

In addition, in a case where the engine rotational speed is reduced to arotational speed lower than the second set rotational speed duringtraveling due to the traveling load, working load or the like, as adecrease amount from the second set rotational speed becomes larger, thechange gear ratio of the continuously variable transmission devicebecomes a larger change gear ratio on the low-speed side, and thus aload on the engine is reduced. Therefore, the engine can be impartedwith a viscous property, to thereby effectively suppress the generationof engine stall which may otherwise be caused by overload.

Then, when the engine rotational speed is increased to or above thesecond set rotational speed, regardless of fluctuation of the enginerotational speed caused by the accelerator operation, traveling load orworking load, the change gear ratio of the continuously variabletransmission device becomes constant at the smallest change gear ratio,and therefore, a relationship in which the vehicle speed is proportionalto the engine rotational speed is retained. Accordingly, with respect tothe work vehicle in which power from the engine whose speed is notchanged by the continuously variable transmission device is generallyused for work, in the case where the work, such as rotary tillage work,is performed in which the vehicle speed and the actuation speed of theworking device are desired to be retained in a relationship in whichthey are proportional to the engine rotational speed, even through theengine rotational speed is changed due to traveling load, working loador the like during work traveling, the vehicle speed and the actuationspeed of the working device are changed in accordance with the change inthe engine rotational speed, while they are retained in the constantrelationship. As a result, an unevenness in the working trail of theworking device can be prevented which may otherwise be caused by thevehicle speed and the actuation speed of the working device not beingretained in a constant relationship.

Furthermore, when the engine rotational speed is a rotational speedbetween the first set rotational speed and the second set rotationalspeed, the engine rotational speed as well as the change gear ratio ofthe continuously variable transmission device is changed by theaccelerator operation, a range of adjustment of the vehicle speed by theaccelerator operation becomes wider, and an amount of change in thevehicle speed relative to the accelerator operation amount becomeslarger, and acceleration and deceleration by the accelerator operationis facilitated. Therefore, moving can be comfortably performed.

(3) A speed change system for a work vehicle having a continuouslyvariable transmission device including: an engine rotational speeddetector configured to detect an engine rotational speed; a vehiclespeed control unit configured to control a change gear ratio of thecontinuously variable transmission device; and a vehicle speed detectorconfigured to detect a vehicle speed; wherein the vehicle speed controlunit computes a control target vehicle speed based on the detectedengine rotational speed and a change gear ratio which is set for thecontinuously variable transmission device, and controls the change gearratio of the continuously variable transmission device so that thedetected vehicle speed matches the control target vehicle speed.

In this system, the vehicle speed control unit performs a vehicle speedfeedback control to change the change gear ratio of the continuouslyvariable transmission device so that the vehicle speed detected by avehicle speed detector matches the control target vehicle speed, andtherefore, an amount of fluctuation in the power transmission efficiencyin the continuously variable transmission device caused by fluctuationin load can be compensated by the control actuation of the vehicle speedcontrol unit, and regardless of the fluctuation of the powertransmission efficiency in the continuously variable transmissiondevice, the change gear ratio of the continuously variable transmissiondevice can be stabilized. Accordingly, the speed change by thecontinuously variable transmission device can be performed with highaccuracy, and a relationship in which the engine rotational speed andthe vehicle speed correspond to each other at the change gear ratio ofthe continuously variable transmission device set by the change gearratio setting unit (for example, the relationship in which the vehiclespeed is proportional to the engine rotational speed) can be retainedwith high accuracy.

As a result, with respect to the work vehicle, such as tractor, in whichpower from the engine whose speed is changed by the transmission devicefor traveling is generally used for traveling and power from the enginewhose speed is not changed by the transmission device for traveling isgenerally used for work, in the case where work, such as rotary tillagework, is performed in which the vehicle speed and the actuation speed ofthe working device are desired to be accurately retained in arelationship in which they are proportional to the engine rotationalspeed, the relationship can be retained with high accuracy regardless ofthe fluctuation of the power transmission efficiency in the continuouslyvariable transmission device, and a difference is unlikely to occurbetween the vehicle speed and an appropriate speed for the actuationspeed of the working device. Thus the working trail of the workingdevice can be made uniform with high accuracy, and the work accuracy ofrotary tillage work or the like can be enhanced.

(4) A speed change system for a work vehicle having a continuouslyvariable transmission device including: a computing unit configured tocompute a rated vehicle speed obtained when an engine rotational speedreaches a rated rotational speed of an engine, based on the ratedrotational speed and a change gear ratio set for the continuouslyvariable transmission device; a display device including: a vehiclespeed display part configured to display information regarding a vehiclespeed; and a speed change step display part for variable speed changeconfigured to display with characters without unit the rated vehiclespeed output by the computing unit as a speed change step of thecontinuously variable transmission device; and a display control unitconfigured to control the display device.

According to this system, when the change gear ratio of the continuouslyvariable transmission device is changed by the change gear ratio settingunit, the speed change step of the continuously variable transmissiondevice displayed on the speed change step display part for countinuouslyvariable speed change is changed in a continuous manner. In other words,a state in which the change gear ratio of the continuously variabletransmission device is changed in a continuous manner can be clearlyindicated. Then, with such a continuous change in the change gear ratioof the continuously variable transmission device, a precise setting ofthe change gear ratio the continuously variable transmission device inaccordance with the work condition is facilitated.

In addition, at the change gear ratio of the continuously variabletransmission device set by the change gear ratio setting unit, the speedchange step of the continuously variable transmission device displayedon the speed change step display part for continuously variable speedchange is a vehicle speed upon a rated rotation obtained when an outputrotational speed of the engine reaches the rated rotational speed, i.e.,the highest speed obtained when the accelerator operation becomesmaximum, and therefore, without using a vehicle speed table which storesthe highest speed for each speed change step, the highest speed for thespeed change step set by the change gear ratio setting unit can beeasily understood. Accordingly, regardless of the state of the vehicleincluding traveling and stopping, the vehicle speed setting inaccordance with the work condition is facilitated.

Therefore, there can be displayed a state in which the change gear ratioof the continuously variable transmission device is changed in acontinuous manner in accordance with the change of the speed change stepby the change gear ratio setting unit, and moreover, information can bedisplayed in such a suitable manner for the work vehicle that thevehicle speed setting in accordance with the work condition isfacilitated, and thus workability can be enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of an entire tractor.

FIG. 2 is a plan view of the entire tractor.

FIG. 3 is a schematic plan view showing a transmission structure of thetractor.

FIGS. 4A, 4B and 4C are block diagrams showing a control configuration.

FIG. 5 is a schematic view showing a configuration and an operationstructure of a main transmission device (HST).

FIG. 6 is a flow chart of vehicle speed control.

FIG. 7 is a flow chart of forward-reverse switching control.

FIG. 8 is a flow chart of start-stop control.

FIG. 9 is a flow chart of braking start-stop control.

FIG. 10 shows graphs of relationship among engine rotational speed,change gear ratio of the main transmission device and vehicle speed.

FIG. 11 is a flow chart of vehicle speed setting control.

FIG. 12 is a flow chart of first switching control.

FIG. 13 is a flow chart of second switching control.

FIG. 14 is a flow chart of third switching control.

FIG. 15 is a front view of a display panel.

FIG. 16 is a front view of a liquid crystal display device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinbelow, as one example of embodiment to carry out the presentinvention, a case in which a speed change system of a work vehicleaccording to the present invention is applied to a tractor as oneexample of the work vehicle will be described with reference to thedrawings.

As shown in FIGS. 1 and 2, the tractor is configured as four-wheel-drivetype including a pair of right and left front wheels 1 which aresteerable and drivable, and a pair of right and left rear wheels 2 whichare drivable and to which braking is independently applicable. A frontportion side of the tractor has an engine 3 mounted thereon, and to arear portion of the engine 3, a transmission case (hereinafter, referredto as “T/M case”) 4, also serving as a body frame, is connected. In arear portion side of the tractor, a steering wheel 5 for front wheelsteering and a driver's seat 6 are disposed to form a boarding operationpart 7, and a gate-shaped protection frame 8 is vertically arranged.

As shown in FIGS. 1 and 3, in the tractor, power from the engine 3 istransmitted to a main transmission device 9 for traveling. Power changedby the main transmission device 9 is used for traveling, and non-changedpower which has passed through the main transmission device 9 is usedfor working. The power for traveling is transmitted to an auxiliarytransmission device 10 for traveling, shifted by the auxiliarytransmission device 10, and divided into power for front wheel drive andpower for rear wheel drive. The power for front wheel drive istransmitted through a power transmission switching device 11 for frontwheel, a differential device 12 for front wheel, and the like, to theright and left front wheels 1. The power for rear wheel drive istransmitted through a differential device 13 for rear wheel and thelike, to the right and left rear wheels 2. The power for working istransmitted through a clutch 14 for working, a transmission device 15for working, and the like, to a PTO shaft 16 for power takeoff.

To sum up, the tractor is configured as clutchless type in which no mainclutch is present which would otherwise connect or disconnect thetransmission from the engine 3 to the right and left front wheels 1 andthe right and left rear wheels 2 as drive wheels, and at the same timethe transmission from the engine 3 to the PTO shaft 16, through theconnection or disconnection of the transmission from the engine 3 to themain transmission device 9.

As shown in FIGS. 1 and 2, on a lower right side of the steering wheel 5in the boarding operation part 7, an accelerator lever 17 is providedwhich is swingable in a front-rear direction and is capable of retainingits position at a desired operation position. In a right foot area, anaccelerator pedal 18 and a pair of right and left brake pedals 19 areprovided, which are configured as pressing operation type toautomatically resume respective press canceling positions. On a leftside of the driver's seat 6, there are provided: a main shift lever 20as one example of a speed change operation tool, which is swingable inthe front-rear direction and capable of retaining its position at adesired operation position; and an auxiliary shift lever 21 which isswingable in the front-rear direction and capable of retaining itsposition at a low-speed position, a high-speed position or a high-speedmovement position. On a left lower side of the steering wheel 5, an FRlever 22 for switching forward-reverse movement is provided which isswingable in the front-rear direction and capable of retaining itsposition at a neutral position, a forward position or a reverseposition. In a left foot area, a stop pedal 23 is provided which isconfigured as pressing operation type to automatically resume a presscanceling position. On a front lower side of the steering wheel 5, adisplay panel 24 is provided which is configured to display information,such as vehicle speed.

As shown in FIGS. 4B and 4C, the engine 3 is configured so that itsoutput rotational speed is non-stepwise adjusted between an idlingrotational speed and a rated rotational speed, by an actuation of anaccelerator cylinder 26 which is configured to operate a speed governinglever 25 of a speed governor (not shown) thereof. An electric cylinderis used for the accelerator cylinder 26. The actuation of theaccelerator cylinder 26 is controlled by a control actuation of anaccelerator control unit 27A provided as control program in anelectronic control unit (hereinbelow, referred to as “ECU”) 27.

The ECU 27 includes a microcomputer having a CPU, an EEPROM and thelike. To the ECU 27 are input: an output of a lever sensor 28 fordetecting an operation position of the accelerator lever 17; an outputof a pedal sensor 29 for detecting an operation position of theaccelerator pedal 18; and an output of a lever sensor 30 for detectingan operation position of the speed governing lever 25. A rotatablepotentiometer is used for each of the lever sensor 28 for acceleratorlever, the pedal sensor 29 for accelerator pedal, and the lever sensor30 for speed governing lever.

The accelerator control unit 27A is configured to compare the operationposition of the accelerator lever 17 and the operation position of theaccelerator pedal 18 based on the output of the lever sensor 28 foraccelerator lever, the output of the pedal sensor 29 for acceleratorpedal, and the output of the lever sensor 30 for speed governing lever,and when the operation position of the accelerator pedal 18 is not on ahigh-speed side relative to the operation position of the acceleratorlever 17, the actuation of the accelerator cylinder 26 is controlled sothat the operation position of the speed governing lever 25 correspondsto the operation position of the accelerator lever 17. When theoperation position of the accelerator pedal 18 is on the high-speed siderelative to the operation position of the accelerator lever 17, theactuation of the accelerator cylinder 26 is controlled so that theoperation position of the speed governing lever 25 corresponds to theoperation position of the accelerator pedal 18.

As shown in FIGS. 3 and 5, a hydrostatic transmission device(hereinbelow, referred to as “HST”) is used for the main transmissiondevice 9. The HST 9 is formed of a variable capacity pump 31 and avariable capacity motor 32, each of which are of axial plunger type, andthe like, built in the T/M case 4. Power (non-changed power; non-changedrotational speed) which has passed through the HST 9 is output to a pumpshaft 31A of the variable capacity pump 31 as power for working, whilepower (rotational speed) changed by the HST 9 is output to a motor shaft32A of the variable capacity motor 32 as power for traveling. Thevariable capacity pump 31 and the variable capacity motor 32 areconnected so as to form a closed circuit through a first oil passage 33and a second oil passage 34. In the closed circuit, oil is supplied froma charge pump 35 actuated by the power from the engine 3.

As shown in FIGS. 4A, 4B, 4C and 5, the variable capacity pump 31 isconfigured to non-stepwise alter an operation angle of a pump swashplate 31B (a swash plate angle of the variable capacity pump 31), by anactuation of a variable speed cylinder 36 for pump provided inside theT/M case 4. For the variable speed cylinder 36, a double-acting typehydraulic cylinder is used, which has a pair of compression springs 37embedded therein and configured to bias the variable speed cylinder 36to return to a neutral state in which the operation angle of the pumpswash plate 31B is zero. The actuation of the variable speed cylinder 36is controlled by actuations of a variable speed valve 38 for forwardtraveling and a variable speed valve 39 for reverse traveling, eachformed of an electromagnetic proportional valve. The actuations of thevariable speed valves 38,39 are controlled by a control actuation of avehicle speed control unit 27B provided as control program in the ECU27.

To sum up, the variable speed cylinder 36 for pump, the variable speedvalve 38 for forward traveling, and the variable speed valve 39 forreverse traveling form a pump swash plate operation unit A which isconfigured to non-stepwise operate the pump swash plate 31B of thevariable capacity pump 31 in accordance with the control actuation ofthe vehicle speed control unit 27B.

The variable capacity motor 32 is configured to be switched between twosteps, including a low-speed step in which an operation angle of a motorswash plate 32B (a swash plate angle of the variable capacity motor 32)is set to a set angle for low-speed and a high-speed step in which theoperation angle is set to a set angle for high-speed, by actuations of alow-speed cylinder 40 for motor and a high-speed cylinder 41 for motor,provided inside the T/M case 4. A single-acting type hydraulic cylinderis used for each of the low-speed cylinder 40 and the high-speedcylinder 41. The actuations of the low-speed cylinder 40 and thehigh-speed cylinder 41 are controlled by an actuation of a variablespeed valve 42 formed of a pilot-operable switching valve. The actuationof the variable speed valve 42 is controlled by an actuation of a speedchange operation valve 43 formed of an electromagnetic proportionalvalve. The actuation of the speed change operation valve 43 iscontrolled by the control actuation of the vehicle speed control unit27B.

To sum up, the low-speed cylinder 40 for motor, the high-speed cylinder41 for motor, the variable speed valve 42 and the speed change operationvalve 43 form a motor swash plate operation unit B which is configuredto operate the motor swash plate 32B of the variable capacity motor 32between two steps including high and low, in accordance with the controlactuation of the vehicle speed control unit 27B.

The pump swash plate operation unit A and the motor swash plateoperation unit B form a change gear ratio altering unit C configured tochange a change gear ratio of the main transmission device 9, and thevehicle speed control unit 27B functions as a swash plate angle controlunit D for controlling an actuation of the change gear ratio alteringunit C. The main transmission device 9 is configured as electroniccontrol type in which the change gear ratio can be non-stepwise andstepwise altered, by altering the operation angles of the pump swashplate 31B and the motor swash plate 32B through the control of thechange gear ratio altering unit C using the control actuation of thevehicle speed control unit 27B.

Though not shown, for the auxiliary transmission device 10, a gear typetransmission device (stepped transmission device) is used which iscapable of three-step speed change, including a low-speed step forlow-speed working, a high-speed step for high-speed working, and afastest step for high-speed moving. The auxiliary transmission device 10is linked to the auxiliary shift lever 21 through a mechanical linkagemechanism for auxiliary speed change, in such a manner that, when theauxiliary shift lever 21 is operated to the low-speed position, a speedchange step is shifted to the low-speed step, when the auxiliary shiftlever 21 is operated to the high-speed position, the speed change stepis shifted to the high-speed step, and when the auxiliary shift lever 21is operated to the high-speed movement position, the speed change stepis shifted to the fastest step.

It should be noted that, in this tractor, a traveling transmissionsystem is configured in such a manner that: when an engine rotationalspeed, the change gear ratio of the main transmission device 9, and thespeed change step of the auxiliary transmission device 10 are set to therated rotational speed, a fastest change gear ratio (smallest changegear ratio), and the low-speed step for low-speed working, respectively,the obtained maximum speed at low-speed working becomes 5 km/h; when theengine rotational speed, the change gear ratio of the main transmissiondevice 9, and the speed change step of the auxiliary transmission device10 are set to the rated rotational speed, the fastest change gear ratio,and the high-speed step for high-speed working, respectively, theobtained maximum speed at high-speed working becomes 12 km/h; and whenthe engine rotational speed, the change gear ratio of the maintransmission device 9, and the speed change step of the auxiliarytransmission device 10 are set to the rated rotational speed, thefastest change gear ratio, and the fastest step for high-speed moving,respectively, the obtained maximum speed at high-speed moving becomes 30km/h.

As shown in FIG. 3, on a right side and a left side in a rear portion ofthe T/M case 4, multiplate type parking brakes 44 each configured to puta brake on the corresponding rear wheel 2 are provided. The left parkingbrake 44 is linked to the left brake pedal 19 and the right parkingbrake 44 is linked to the right brake pedal 19 through the correspondingmechanical linkage mechanism for braking (not shown), in such a mannerthat a brake is put on the corresponding rear wheel 2, with a brakingforce in accordance with a pressing operation amount of thecorresponding brake pedal 19.

With this configuration, for example, during a turning traveling inwhich the steering wheel 5 is operated to turn in a turning direction,by performing a pressing operation on one of the right and left brakepedals 19 alone corresponding to the rear wheel 2 on an inner side ofthe turn, a turning state at that moment can be switched from a normalturning state caused by the turning operation of the steering wheel 5 toa braking turning state in which, in addition to the normal turningstate, a brake is put on the rear wheel 2 on the inner side of the turn,and thus a turning radius of a vehicle body can be made small. Inaddition, during a straight-ahead traveling, if a deviation occurs in atraveling direction of the vehicle body caused by roughness in anagricultural field plough pan or the like, by performing a pressingoperation on the brake pedal 19 on an opposite side to a direction ofthe deviation, a traveling state at that moment can be switched from astraight-ahead traveling state to a single braking traveling state.Accordingly, while retaining the steering operation system in thestraight-ahead traveling state, the traveling direction of the vehiclebody can be corrected. Then, by performing a dual pressing operation ofthe right and left brake pedals 19, the right and left parking brakes 44can be used as a brake for deceleration stop.

Though not shown, the right and left brake pedals 19 are provided with aconnection mechanism switchable between a connecting state in which theright and left brake pedals 19 are connected to prevent an independentoperation of one of the right and left brake pedals 19, and a connectioncanceling state in which the connection of the right and left brakepedals 19 is cancelled to allow the independent operation of one of theright and left brake pedals 19. Accordingly, when the speed change stepof the auxiliary transmission device 10 is set to the low-speed step orthe high-speed step for working, by canceling in advance the connectionbetween the right and left brake pedals 19 through the connectionmechanism, it becomes possible to switch the state to the single brakingtraveling state or the braking turning state, which is useful duringwork traveling. When the speed change step of the auxiliary transmissiondevice 10 is set to the fastest step for high-speed moving, byconnecting in advance the right and left brake pedals 19 through theconnection mechanism, the traveling state is prevented from beingswitched to the single braking traveling state or the braking turningstate, which may otherwise be occurred by a failure of stepping on thebrake pedal 19 during high-speed moving.

As shown in FIG. 5, the stop pedal 23 is linked to an unloading valve 45provided in the main transmission device 9 through a mechanical linkagemechanism for stopping a traveling (not shown). The unloading valve 45is provided on a return oil passage 46 connected to the first oilpassage 33 in the main transmission device 9, and is configured in sucha manner that, when the stop pedal 23 is not pressed to a predeterminedpress limit region, a cut-off state is retained in which oil isprevented from being discharged from the first oil passage 33, and inconjunction with a pressing operation of the stop pedal 23 to thepredetermined press limit region, the state is switched to acommunicating state in which the oil is allowed to be discharged fromthe first oil passage 33.

In other words, by performing the pressing operation of the stop pedal23 to the predetermined press limit region, the oil can be dischargedfrom the first oil passage 33 of the main transmission device 9 so as tocut off the transmission of a hydraulic pressure from the variablecapacity pump 31 to the variable capacity motor 32, to thereby stop orprevent the traveling of the vehicle body.

As a result, when need arises to stop the vehicle body in the singlebraking traveling state or the braking turning state in which one of theright and left brake pedals 19 is pressed, by performing the pressingoperation of the stop pedal 23 to the predetermined press limit region,the vehicle body can be rapidly stopped without an effort of switchingof the right and left brake pedals 19 from a single pressing state to adual pressing state.

As shown in FIGS. 1 and 4C, in the rear portion of the T/M case 4, thereare provided: a pair of right and left lift arms 47 which allow liftingand lowering operations of a working device (not shown), such as rotarytilling device and agent sparging device, connected to a rear portion ofthe tractor; a pair of right and left lift cylinders 48 configured toswing-drive the respective lift arms 47; and the like. A single-actingtype hydraulic cylinder is used for each of the right and left liftcylinders 48. When the working device, such as rotary tilling deviceconfigured to be actuated by power from the tractor, is connected to therear portion of the tractor, power taken out from the PTO shaft 16 canbe supplied to the working device through a removable externaltransmission device (not shown).

Referring to FIGS. 2 to 4C, the clutch 14 for working is switchablebetween an on-state in which non-changed power (non-changed rotationalspeed) from the main transmission device 9 is transmitted to the PTOshaft 16 through the transmission device 15 for working and an off-statein which the transmission is cut off, by an actuation of a clutch valve49 formed of an electromagnetic control valve. The actuation of theclutch valve 49 is controlled by a control actuation of a working powercontrol unit 27C provided as control program in the ECU 27. A multiplatetype hydraulic clutch is used for the clutch 14 for working.

To the ECU 27 are input: an output of a PTO switch 50 for setting anon-off of the transmission to the PTO shaft 16; an output of an armsensor 51 for detecting a swing angle of the lift arm 47 as a height ofthe working device; and the like, and the working power control unit 27Cis configured to control the actuation of the clutch valve 49, inaccordance with these outputs and the like.

Specifically, the working power control unit 27C is configured to detectan operation position of the PTO switch 50 based on the output of thePTO switch 50. When the operation position of the PTO switch 50 is anoff-position, the actuation of the clutch valve 49 is controlled in sucha manner that the clutch 14 for working is retained in the off-state.When the operation position of the PTO switch 50 is an on-position, theactuation of the clutch valve 49 is controlled in such a manner that theclutch 14 for working is retained in the on-state. When the operationposition of the PTO switch 50 is an automatic position, the actuation ofthe clutch valve 49 is controlled based on the output of the arm sensor51 for lift arm, in such a manner that the clutch 14 for working isretained in the off-state while the swing angle of the lift arm 47 staysat a set angle or in an upper side angle region above the set angle, andthe clutch 14 for working is retained in the on-state while the swingangle of the lift arm 47 stays in a ground side angle region below theset angle.

In other words, in the case where the working device of a ground worktype, such as rotary tilling device, configured to be actuated by powerfrom the PTO shaft 16 is connected, by operating in advance the PTOswitch 50 to the automatic position, the working device can be actuatedin conjunction with the lowering operation of the working device thatmakes the swing angle of the lift arm 47 less than the set angle. Inaddition, an actuation of the working device can be stopped inconjunction with the lifting operation of the working device that makesthe swing angle of the lift arm 47 equal to or above the set angle.

The PTO switch 50 is disposed on a right side of the driver's seat 6 inthe boarding operation part 7 (see FIG. 2). For the PTO switch 50, adial type multicontact switch is used which is 3-position-switchable andconfigured to resume the off-position by a downward pressing operation.A rotatable potentiometer is used for the arm sensor 51 for lift arm.

Though not shown, for the transmission device 15 for working, a geartype transmission device is used which is switchable between a normalrotation and a reverse rotation, and is capable of four-step speedchange for the normal rotation. The transmission device 15 for workingis linked to a shift lever 52 for working through a mechanical linkagemechanism for working speed change, in such a manner that an actuationstate is switchable in accordance with an operation position of theshift lever 52 for working (see FIG. 2) provided on the right side ofthe driver's seat 6 in the boarding operation part 7. The shift lever 52for working is of a combined swingable type in which a swingingoperation can be made in the front-rear directions and right-leftdirections, and is capable of retaining its position.

Referring to FIGS. 4A, 4B and 4C, actuations of the right and left liftcylinders 48 is controlled by an actuation of a lifting valve 53 formedof an electromagnetic control valve. The actuation of the lifting valve53 is controlled by a control actuation of a lifting-lowering controlunit 27D provided as control program in the ECU 27.

To the ECU 27 are input: an output of a lever sensor 55 for detecting anoperation position of a height setting lever 54 as control target heightof the working device; an output of a lever sensor 57 for detecting anoperation of a lifting command lever 56 from a neutral position to alifting position or lowering position; an output of an upper limitsetter 58 for outputting a turning operation amount from a standardposition as a control target upper limit position of the working device;and the like. The lifting-lowering control unit 27D is configured toperform, based on these outputs and the like: a height control forpositioning the working device at a desired height position; a liftingcontrol, in priority to the height control, for lifting the workingdevice to a set upper limit position; and the like.

In the height control, based on the output of the lever sensor 55 forheight setting lever and the output of the arm sensor 51 for lift arm,the actuation of the lifting valve 53 is controlled in such a mannerthat the swing angle of the lift arm 47 corresponds to the operationposition of the height setting lever 54.

The lifting control is performed in priority to the height control, whenthe lever sensor 57 detects the operation of the lifting command lever56 to the lifting position. In the lifting control, based on the outputof the upper limit setter 58 and the output of the arm sensor 51 forlift arm, the actuation of the lifting valve 53 is controlled in such amanner that the swing angle of the lift arm 47 corresponds to theturning operation amount from the standard position of the upper limitsetter 58. When the lever sensor 57 detects the operation of the liftingcommand lever 56 to the lowering position, the priority given to thelifting control is cancelled and the height control is performed.

In other words, by an operation of the height setting lever 54, theheight of the working device can be altered to a desired height inaccordance with the operation position of the height setting lever 54.In addition, by a swinging operation of the lifting command lever 56 tothe lifting position, the height of the working device can be altered tothe upper limit position set by the upper limit setter 58. By theswinging operation of the lifting command lever 56 to the loweringposition, the height of the working device can be returned to thedesired height in accordance with the operation position of the heightsetting lever 54.

As shown in FIGS. 1 and 2, the height setting lever 54 is provided onthe right side of the driver's seat 6 in the boarding operation part 7,and is swingable in the front-rear direction and capable of retainingthe position. The lifting command lever 56 is disposed on the lowerright side of the steering wheel 5 in the boarding operation part 7, andis vertically swingable and capable of returning to the neutralposition. The upper limit setter 58 is provided on the right side of thedriver's seat 6 in the boarding operation part 7, and is configured as adial operation type. A rotatable potentiometer is used for each of thelever sensor 55 for height setting lever and the upper limit setter 58.A switch is used for the lever sensor 57 for lifting command lever.

Meanwhile, with respect to the HST 9 forming the main transmissiondevice 9, it has been known that as a load becomes large, a loss of ahydraulic pressure due to oil leakage becomes large, leading to areduction in power transmission efficiency. For example, a swash plateangle sensor may be provided for detecting the operation angle of thepump swash plate 31B in the main transmission device 9, and the vehiclespeed control unit 27B may be configured to perform a swash platefeedback control in which an actuation of the pump swash plate operationunit A is controlled in such a manner that an output of the swash plateangle sensor matches a control target operation angle which is set basedon an operation position of the main shift lever 20 (an operation angleof the swash plate angle sensor falls on a dead band of the controltarget operation angle). In this case, the operation angle of the pumpswash plate 31B can be matched with the control target operation angleby the swash plate feedback control, but a fluctuation of the powertransmission efficiency in the main transmission device 9 caused by loadfluctuation cannot be compensated. As a result, the fluctuation of thepower transmission efficiency hinders stability of the change gear ratioof the main transmission device 9, and it becomes difficult toaccurately retain a relationship in which the vehicle speed isproportional to the engine rotational speed.

Accordingly, in the case where work, such as rotary tillage work, isperformed in which the vehicle speed and an actuation speed of theworking device are desired to be accurately retained in a relationshipin which they are proportional to the engine rotational speed, since therelationship cannot be retained with high accuracy, a difference islikely to occur between an appropriate vehicle speed for the actuationspeed of the working device and the vehicle speed, leading to anunevenness in a working trail of the working device, and thus it becomesdifficult to make the working trail of the working device uniform withhigh accuracy.

Therefore, in a vehicle speed control in which the vehicle speed of thetractor is controlled, the vehicle speed control unit 27B is configuredto perform a vehicle speed feedback control in which the actuation ofthe change gear ratio altering unit C is controlled in such a mannerthat the vehicle speed matches a control target vehicle speed which isset based on the operation position of the main shift lever 20 or thelike (the vehicle speed falls on a dead band of the control targetvehicle speed).

Specifically stating the configuration, referring to FIGS. 3, 4A, 4B, 4Cand 6, to the ECU 27 are input: an output of a lever sensor 59 asoperation position detector for detecting the operation position of themain shift lever 20; an output of a lever sensor 60 for detecting anoperation position of the auxiliary shift lever 21; an output of a leversensor 61 for detecting an operation position of the FR lever 22; anoutput of an engine sensor (engine rotational speed detector) 62 fordetecting the engine rotational speed; an output of a vehicle speedsensor (vehicle speed detector) 63 for detecting an output rotationalspeed of the main transmission device 9 as vehicle speed upon control;an output of a pedal sensor 64 for detecting accession of the stop pedal23 to the predetermined press limit region; and an output of a pedalsensor 65 for detecting a dual pressing operation amount of the rightand left brake pedals 19 from the press canceling position. In addition,the ECU 27 is provided with vehicle speed setting data showing arelationship among the engine rotational speed, the change gear ratio ofthe main transmission device 9, and the vehicle speed (the outputrotational speed of the main transmission device 9).

A rotatable potentiometer is used for each of the lever sensor 59 formain shift lever and the pedal sensor 65 for brake pedal. A switch isused for each of the lever sensor 60 for auxiliary shift lever, thelever sensor 61 for FR lever, and the pedal sensor 64 for stop pedal. Anelectromagnetically pick up type rotation sensor is used for each of theengine sensor 62 and the vehicle speed sensor 63. Map data, relationalexpression or the like is used for the vehicle speed setting data.

In the vehicle speed feedback control, first the control target vehiclespeed (control target rotational speed) for an output of the vehiclespeed sensor 63 is read out which is determined by a vehicle speedsetting control based on: the change gear ratio of the main transmissiondevice 9 (hereinbelow, referred to as “set change gear ratio”) set bythe main shift lever 20 from the output of the lever sensor 59 for mainshift lever; the speed change step of the auxiliary transmission device10 (hereinbelow, referred to as “set speed change step”) set by theauxiliary shift lever 21 from the output of the lever sensor 60 forauxiliary shift lever; the output of the engine sensor 62; and thevehicle speed setting data (step #601), and whether the control targetvehicle speed is for forward traveling or for reverse traveling from theoutput of the lever sensor 61 for FR lever is determined (step #602).Next, the output of the vehicle speed sensor 63 is read (step #603), andthe control target vehicle speed for forward traveling or for reversetraveling is compared with the output of the vehicle speed sensor 63(steps #604, #605). When the output of the vehicle speed sensor 63matches the control target vehicle speed for forward traveling or forreverse traveling (the vehicle speed upon control which is output by thevehicle speed sensor 63 falls on the dead band of the control targetvehicle speed for forward traveling or for reverse traveling), aconstant-speed control is performed which controls an electricalcontinuity to the variable speed valve 38 for forward traveling or thevariable speed valve 39 for reverse traveling and the speed changeoperation valve 43 so as to retain the current vehicle speed (step#606). When the output of the vehicle speed sensor 63 is smaller thanthe control target vehicle speed for forward traveling or for reversetraveling, an acceleration control is performed which controls theelectrical continuity to the variable speed valve 38 for forwardtraveling or the variable speed valve 39 for reverse traveling and thespeed change operation valve 43 to accelerate the vehicle speed, so thatthe output of the vehicle speed sensor 63 matches the control targetvehicle speed for forward traveling or for reverse traveling (step#607). When the output of the vehicle speed sensor 63 is larger than thecontrol target vehicle speed for forward traveling or for reversetraveling, a deceleration control is performed which controls theelectrical continuity to the variable speed valve 38 for forwardtraveling or the variable speed valve 39 for reverse traveling and thespeed change operation valve 43 to decelerate the vehicle speed, so thatthe output of the vehicle speed sensor 63 matches the control targetvehicle speed for forward traveling or for reverse traveling (step#608).

With this configuration, solely the fluctuation of the powertransmission efficiency in the main transmission device (HST) 9, whichis caused by the loss of the hydraulic pressure that fluctuates alongwith the increase/decrease of the load, can be compensated by thecontrol actuation of the vehicle speed control unit 27B, and thus thespeed change by the main transmission device 9 can be performed withhigh accuracy. As a result, in the case where the work, such as rotarytillage work, is performed in which the vehicle speed and the actuationspeed of the working device are desired to be accurately retained in arelationship in which they are proportional to the engine rotationalspeed, the relationship can be retained with high accuracy regardless ofthe fluctuation of the power transmission efficiency in the maintransmission device 9, and a difference is unlikely to occur between theappropriate speed for the actuation speed of the working device and thevehicle speed. Thus the working trail of the working device can be madeuniform with high accuracy.

As shown in FIGS. 4A, 4B, 4C and 7, the vehicle speed control unit 27Bis configured to perform a forward-reverse switching control based on anoperation of the FR lever 22, when the operation of the FR lever 22 isdetected from the output of the lever sensor 61 for FR lever.

Hereinbelow, the forward-reverse switching control will be describedwith reference to the flow chart of FIG. 7. When the operation of the FRlever 22 is detected from the output of the lever sensor 61 for FRlever, the operation position of the FR lever 22 after the operation isalso detected (steps #701, #702). When the operation position of the FRlever 22 after the operation is the neutral position, the electricalcontinuity to the variable speed valve 38 for forward traveling and thevariable speed valve 39 for reverse traveling is disconnected (step#703). Accordingly, the operation angle of the pump swash plate 31B canbe zeroed to thereby make the main transmission device 9 neutral, andregardless of the control target vehicle speed determined by the vehiclespeed setting control, the output of the vehicle speed sensor 63 can bezeroed. When the operation position of the FR lever 22 after theoperation is the forward position or the reverse position, it isdetermined whether or not a predetermined driving condition isestablished by this operation to the forward position or the reverseposition (steps #704, #705). When the predetermined driving condition isestablished, the process advances to the vehicle speed feedback control(step #706). When the predetermined driving condition is notestablished, the disconnection of the electrical continuity to thevariable speed valve 38 for forward traveling and the variable speedvalve 39 for reverse traveling is continued (step #707).

It should be noted that, a state in which “the predetermined drivingcondition is established” means a state in which the lever sensor 61 forFR lever detects that the operation position of the FR lever 22 is theforward position or the reverse position, the pedal sensor 64 for stoppedal does not detect the accession of the stop pedal 23 to thepredetermined press limit region, and the pedal sensor 65 for brakepedal detects that both pressing operation positions of the right andleft brake pedals 19 are within a braking canceling region.

As shown in FIGS. 4A, 4B, 4C and 8, the vehicle speed control unit 27Bis configured to perform a start-stop control based on the operation ofthe stop pedal 23, when the operation of the stop pedal 23 is detectedfrom the output of the pedal sensor 64 for stop pedal.

Hereinbelow, the start-stop control will be described with reference tothe flow chart of FIG. 8. When the operation of the stop pedal 23 isdetected from the output of the pedal sensor 64 for stop pedal, theoperation position of the stop pedal 23 after the operation is alsodetected (step #801). When the operation position of the stop pedal 23after the operation is within the predetermined press limit region, theelectrical continuity to the variable speed valve 38 for forwardtraveling and the variable speed valve 39 for reverse traveling isdisconnected (step #802). Accordingly, the operation angle of the pumpswash plate 31B can be zeroed to thereby make the main transmissiondevice 9 neutral, and regardless of the control target vehicle speeddetermined by the vehicle speed setting control, the output of thevehicle speed sensor 63 can be zeroed. When the operation position ofthe stop pedal 23 after the operation is outside the press limit region,it is determined whether or not the predetermined driving condition isestablished by this operation (step #803). When the predetermineddriving condition is established, it is determined whether or not a settime has elapsed which is required for the unloading valve 45 to switchfrom the communicating state to a cut-off state in conjunction with theoperation of the stop pedal 23 to outside the press limit region, andfor a circuit pressure of the main transmission device 9 to reach a setvalue (step #804). After the set time has elapsed, the process advancesto the vehicle speed feedback control (step #805). In the step #803,when the predetermined driving condition is not established, thedisconnection of the electrical continuity to the variable speed valve38 for forward traveling and the variable speed valve 39 for reversetraveling is continued (step #806).

It should be noted that, in this start-stop control, the state in which“the predetermined driving condition is established” is the same stateas described for the forward-reverse switching control, in which thepredetermined driving condition is established.

As shown in FIGS. 4A, 4B, 4C and 9, the vehicle speed control unit 27Bis configured to perform a braking start-stop control based on the dualpressing operation of the right and left brake pedals 19, when the dualpressing operation of the right and left brake pedals 19 is detectedfrom the output of the pedal sensor 65 for brake pedal.

Hereinbelow, the braking start-stop control will be described withreference to the flow chart of FIG. 9. When the dual pressing operationof the right and left brake pedals 19 is detected from the output of thepedal sensor 65 for brake pedal, it is determined whether or not thepredetermined driving condition is established (step #901). When thepredetermined driving condition is established, a dual pressingoperation position of the right and left brake pedals 19 is detectedfrom the output of the pedal sensor 65 for brake pedal (steps #902,#903). When the dual pressing operation position of the right and leftbrake pedals 19 is in the braking canceling region, the vehicle speedfeedback control is continued (step #904). When the dual pressingoperation position of the right and left brake pedals 19 is in a brakingspeed change region which is continuous to the braking canceling region,the control target vehicle speed determined by the vehicle speed settingcontrol is read (step #905), and it is detected whether the controltarget vehicle speed is for forward traveling or for reverse travelingfrom the output of the lever sensor 61 for FR lever (step #906). In thiscase, a braking speed change control is performed in which theelectrical continuity to the variable speed valve 38 for forwardtraveling or the variable speed valve 39 for reverse traveling iscontrolled to alter the operation angle of the pump swash plate 31B sothat, as the dual pressing operation amount of the right and left brakepedals 19 in the braking speed change region becomes large, the outputof the vehicle speed sensor 63 approaches zero from the control targetvehicle speed for forward traveling or for reverse traveling (step#907). When the dual pressing operation position of the right and leftbrake pedals 19 is in a braking stop region which is continuous to thebraking speed change region, the electrical continuity to the variablespeed valve 38 for forward traveling and the variable speed valve 39 forreverse traveling is disconnected (step #908). Accordingly, theoperation angle of the pump swash plate 31B can be zeroed to therebymake the main transmission device 9 neutral, and regardless of thecontrol target vehicle speed determined by the vehicle speed settingcontrol, the output of the vehicle speed sensor 63 can be zeroed. Whenthe predetermined driving condition is not established in the step #901,it is determined whether or not the dual pressing operation of the rightand left brake pedals 19 from the braking stop region to the brakingspeed change region is detected (step #909). When the dual pressingoperation of the right and left brake pedals 19 from the braking stopregion to the braking speed change region is detected, it is determinedwhether or not the predetermined driving condition is established bythis operation (step #910). When the predetermined driving condition isestablished, the process advances to the step #902. Accordingly, whenthe dual pressing operation position of the right and left brake pedals19 is in the braking speed change region, the braking speed changecontrol is performed, and as the right and left brake pedals 19 returnto the braking canceling region, the vehicle speed feedback control isperformed. When the dual pressing operation of the right and left brakepedals 19 from the braking stop region to the braking speed changeregion is not detected in the step #909, and when the predetermineddriving condition is not established in the step #910, the disconnectionof the electrical continuity to the variable speed valve 38 for forwardtraveling and the variable speed valve 39 for reverse traveling iscontinued (step #911).

It should be noted that, in this braking start-stop control, the statein which “the predetermined driving condition is established” is thesame state as described for the forward-reverse switching control andthe start-stop control, in which the predetermined driving condition isestablished.

Though not shown, the working power control unit 27C is configured insuch a manner that, when the operation of the stop pedal 23 to thepredetermined press limit region is detected based on the output of thepedal sensor 64 for stop pedal, the working power control unit 27Cdetermines a state of the clutch 14 for working based on the operationposition of the PTO switch 50, the output of the arm sensor 51 for liftarm and the like. When the clutch 14 for working is in the off-state,this state is retained, and when the clutch 14 for working is in theon-state, the clutch 14 for working is switched to the off-state after aset time has elapsed since the detection of the operation of the stoppedal 23 to the predetermined press limit region, so that the actuationof the working device is stopped after the traveling of the vehicle bodyis stopped, for example by switching of the unloading valve 45 to thecommunicating state based on the pressing operation of the stop pedal23.

In addition, when the operation of the stop pedal 23 to the press limitregion is not detected and at the same time the establishment of thepredetermined driving condition is detected, it is determined whether ornot a condition required for switching the clutch 14 for working fromthe off-state to the on-state (hereinbelow, referred to as “workinitiation condition”) is established, based on the operation positionof the PTO switch 50, the output of the arm sensor 51 for lift arm andthe like. When the work initiation condition is established, the clutch14 for working is switched to the on-state, and when the work initiationcondition is not established, the clutch 14 for working is retained inthe off-state.

The working power control unit 27C is configured in such a manner that,when a movement of the right and left brake pedals 19 to a work stopregion set on a press limit side of the braking stop region is detectedbased on the output of the pedal sensor 65 for brake pedal, the workingpower control unit 27C determines the state of the clutch 14 for workingbased on the operation position of the PTO switch 50, the output of thearm sensor 51 for lift arm and the like. When the clutch 14 for workingis in the off-state, this state is retained, and when the clutch 14 forworking is in the on-state, the clutch 14 for working is switched to theoff-state.

When the movement of the right and left brake pedals 19 from the workstop region to the braking stop region outside the work stop region isdetected, it is determined whether or not the state allows thepredetermined driving condition to be established by a subsequentmovement of the right and left brake pedals 19 to the braking cancelingregion. When the state allows the predetermined driving condition to beestablished, it is determined whether or not the work initiationcondition is established. When the work initiation condition isestablished, the clutch 14 for working is switched to the on-state, andwhen the work initiation condition is not established, the clutch 14 forworking is retained in the off-state.

Due to the control actuations of the vehicle speed control unit 27B andthe working power control unit 27C described above, while configured asclutchless type with no main clutch, the transmission to the right andleft front wheels 1 as drive wheels and the right and left rear wheels2, as well as the transmission to the PTO shaft 16, can be connected anddisconnected, by performing the pressing operation of the stop pedal 23or the dual pressing operation of the right and left brake pedals 19.When the traveling of the vehicle body and the actuation of the workingdevice are controlled by the pressing operation of the stop pedal 23 orthe dual pressing operation of the right and left brake pedals 19, theactuation of the working device is stopped after the traveling of thevehicle body is stopped, and the traveling of the vehicle body isstarted after the actuation of the working device is initiated. Withthis configuration, appropriate work can be surely performed at a stopposition and a start position of the vehicle body, and especially when arotary tilling device is implemented as the working device, the unplowedremains at the stop position and the start position of the vehicle bodycan be prevented from being generated.

Though not shown, the pedal sensor 65 for brake pedal is linked to theright and left brake pedals 19 through a mechanical linkage mechanism(not shown) for detecting the dual pressing operation amount. Themechanical linkage mechanism for detecting the dual pressing operationamount is provided with a moving member biased by a spring so as to bedisplaced in a pedal pressing direction in conjunction with the dualpressing operation of the right and left brake pedals 19. The mechanicallinkage mechanism is configured in such a manner that, when a singlepressing operation is performed on one of the right and left brakepedals 19, a displacement of the moving member to follow said one of thebrake pedals 19 is prevented by bringing the moving member into contactwith the other of the brake pedals 19, and that only when the dualpressing operation of the right and left brake pedals 19 is performed,the moving member is allowed to be displaced to follow the right andleft brake pedals 19, and the dual pressing operation amount of theright and left brake pedals 19 at that moment is transmitted to thepedal sensor 65 for brake pedal.

As shown in FIG. 10, there are two types of the vehicle speed settingdata stored in the ECU 27: the first vehicle speed setting data for worktraveling (see FIG. 10 (A)) and the second vehicle speed setting datafor high-speed moving (see FIG. 10 (B)). In each vehicle speed settingdata, a relationship among the engine rotational speed, the change gearratio of the main transmission device 9 and the vehicle speed (theoutput of the vehicle speed sensor 63) is set, using the fastest changegear ratio of the main transmission device 9 corresponding to a fastestposition of the main shift lever 20 as criterion.

Each type of vehicle speed setting data will be described in detailbelow. Basically, both the first and second vehicle speed setting dataare set in the following manner. When the engine rotational speed is afirst set rotational speed N1 which is set to the idling rotationalspeed, it is set in such a manner that the change gear ratio of the maintransmission device 9 becomes a first change gear ratio which is a largechange gear ratio on a low-speed side, and the vehicle speed becomes alow speed which has been set in advance. When the engine rotationalspeed is in a high rotational speed region NH which is equal to or abovea second set rotational speed N2 and is set on a high-speed siderelative to the first set rotational speed N1, it is set in such amanner that the change gear ratio of the main transmission device 9 isretained at the fastest change gear ratio, and the vehicle speed becomesproportional to the engine rotational speed. When the engine rotationalspeed is in a medium rotational speed region NM between the first setrotational speed N1 and the second set rotational speed N2, it is set insuch a manner that, as the engine rotational speed at that momentbecomes low, the change gear ratio of the main transmission device 9 isaltered to larger change gear ratio on the low-speed side between thefirst change gear ratio and the fastest change gear ratio, and thevehicle speed becomes proportional to the engine rotational speed andthe change gear ratio of the main transmission device 9 (the rate ofchange of the vehicle speed becomes constant and larger than the rate ofchange in the high rotational speed region NH). When the enginerotational speed is in a low rotational speed region NL which is belowthe first set rotational speed N1, it is set in such a manner that thechange gear ratio of the main transmission device 9 is altered inaccordance with the engine rotational speed so that the maintransmission device 9 does not become neutral and the vehicle speed doesnot become zero (the rate of change of the vehicle speed becomesconstant and smaller than the rate of change in the high rotationalspeed region NH), as long as the engine rotational speed is larger thanzero.

Specifically, in the first vehicle speed setting data, the first setrotational speed N1 is set to the idling rotational speed as describedabove, and the second set rotational speed N2 is set to a rotationalspeed at which the engine 3 exerts the maximum torque. A relationshipamong the engine rotational speed, the change gear ratio of the maintransmission device 9 and the vehicle speed is set in the followingmanner. In the high rotational speed region NH, the relationship is setin which the vehicle speed is proportional to the engine rotationalspeed so that the vehicle speed corresponds to the engine rotationalspeed at the fastest change gear ratio of the main transmission device9. In the medium rotational speed region NM, the relationship is set inwhich the vehicle speed is proportional to the engine rotational speedand to the change gear ratio of the main transmission device 9 so that,when the engine rotational speed reaches the first set rotational speed(idling rotational speed) N1, the vehicle speed becomes a very low setspeed, and when the engine rotational speed reaches the second setrotational speed N2 at which the maximum torque is exerted, the vehiclespeed corresponds to the engine rotational speed at the fastest changegear ratio of the main transmission device 9. In the low rotationalspeed region NL, the relationship is set in which the vehicle speedcorresponds to the engine rotational speed at the change gear ratio ofthe main transmission device 9 which is inversely proportional to theengine rotational speed so that the vehicle speed becomes constant at avery low set speed (see FIG. 10 (A)).

On the other hand, in the second vehicle speed setting data, the firstset rotational speed N1 is set to the idling rotational speed asdescribed above, and the second set rotational speed N2 is set to arotational speed slightly smaller than the rated rotational speed of theengine 3. The relationship among the engine rotational speed, the changegear ratio of the main transmission device 9 and the vehicle speed isset in the following manner. In the high rotational speed region NH, therelationship is set in which the vehicle speed is proportional to theengine rotational speed so that the vehicle speed corresponds to theengine rotational speed at the fastest change gear ratio of the maintransmission device 9. In the medium rotational speed region NM, therelationship is set in which the vehicle speed is proportional to theengine rotational speed and to the change gear ratio of the maintransmission device 9 so that, when the engine rotational speed reachesthe first set rotational speed (idling rotational speed) N1, the vehiclespeed becomes a low set speed, and when the engine rotational speedreaches the second set rotational speed N2 which is close to andslightly smaller than the rated rotational speed, the vehicle speedcorresponds to the engine rotational speed at the fastest change gearratio of the main transmission device 9. In the low rotational speedregion NL, the relationship is set in which the vehicle speedcorresponds to the engine rotational speed at the change gear ratio ofthe main transmission device 9 which is proportional to the enginerotational speed so that, when the engine rotational speed reaches thefirst set rotational speed (idling rotational speed) N1, the vehiclespeed becomes a low set speed, and when the engine rotational speed iszero, the main transmission device 9 becomes neutral and the vehiclespeed becomes zero (see FIG. 10 (B)).

It should be noted that a dashed-dotted line shown in FIG. 10 shows animaginary condition in which the change gear ratio of the maintransmission device 9 is retained as the fastest change gear ratio inthe low rotational speed region NL and the medium rotational speedregion NM.

In addition, a broken line shown in FIG. 10 (B) shows an imaginarychange gear ratio of the main transmission device 9 which is obtainedwhen the operation angle of the pump swash plate 31B of the variablecapacity pump 31 is altered from zero to an operation limitation anglein proportion to the engine rotational speed which is changed from zeroto the rated rotational speed, while the variable capacity motor 32 isretained at the low-speed step. In other words, the change gear ratio ofthe main transmission device 9 in the low rotational speed region NL ofthe second vehicle speed setting data is a change gear ratio obtained inthe low rotational speed region NL, by altering the operation angle ofthe pump swash plate 31B of the variable capacity pump 31 in proportionto the engine rotational speed while the variable capacity motor 32 isset to the low-speed step.

The vehicle speed control unit 27B is configured to change the vehiclespeed setting data to be adopted, based on the output of the leversensor 60 for auxiliary shift lever functioning as speed change stepdetector for detecting the speed change step of the auxiliarytransmission device (stepped transmission device) 10. Specifically, whenthe speed change step of the auxiliary transmission device 10 which isread out based on the output of the lever sensor 60 for auxiliary shiftlever is the low-speed step for low-speed working or the high-speed stepfor high-speed working, the vehicle speed control unit 27B adopts thefirst vehicle speed setting data, and when the fastest step forhigh-speed moving, adopts the second vehicle speed setting data.

It should be noted that a medium-speed set region of the main shiftlever 20 is a region in which the main shift lever 20 can set the changegear ratio of the main transmission device 9 that can be attained byswitching a speed change step of the variable capacity motor 32 toeither the low-speed step or the high-speed step. It should also benoted that the speed that can be attained at the low-speed step of thevariable capacity motor 32 is defined as a speed obtained based on: achange gear ratio obtained by, while the speed change step of thevariable capacity motor 32 is set to the low-speed step, altering theoperation angle of the pump swash plate 31B of the variable capacitypump 31 from zero to the operation limitation angle; and the enginerotational speed from the second vehicle speed setting data,corresponding to this change gear ratio. A limit value of this speed isdefined as a speed obtained based on: the fastest change gear ratio atthe low-speed step obtained by, while the speed change step of thevariable capacity motor 32 is set to the low-speed step, altering theoperation angle of the pump swash plate 31B of the variable capacitypump 31 to the operation limitation angle; and the engine rotationalspeed from the second vehicle speed setting data, corresponding to thisfastest change gear ratio.

The ECU 27 is provided with a gain for correcting each vehicle speedsetting data which is set using the fastest change gear ratio of themain transmission device 9 as criterion, to the vehicle speed settingdata for which a set change gear ratio corresponding to the operationposition of the main shift lever 20 arbitrarily set is used ascriterion.

The vehicle speed control unit 27B is configured to read out a gaincorresponding to the operation position of the main shift lever 20 fromthe output of the lever sensor 59 for main shift lever, to multiply theadopted vehicle speed setting data by the gain read out, and toautomatically correct the adopted vehicle speed setting data to anappropriate one for which the set change gear ratio corresponding to theoperation position of the main shift lever 20 is used as criterion.

For example, when the operation position of the main shift lever 20 ispositioned halfway (50%) to the fastest position of the main shift lever20, the adopted vehicle speed setting data is multiplied by 0.5, tothereby automatically correct the adopted vehicle speed setting data toan appropriate one for which, throughout its range, a change gear ratiowhich is half (50%) of the fastest change gear ratio of the maintransmission device 9 is used as criterion (data with which arelationship depicted with an imaginary line (dashed-two dotted line) inFIG. 10 is obtained). For example, when the operation position of themain shift lever 20 is the fastest position, the adopted vehicle speedsetting data is multiplied by 1.0, to thereby automatically correct theadopted vehicle speed setting data to an appropriate one for which,throughout its range, the fastest change gear ratio of the maintransmission device 9 is used as criterion (data with which arelationship depicted with a solid line in FIG. 10 is obtained).

Accordingly, as shown in FIG. 10 (A), in the corrected first vehiclespeed setting data, the relationship among the engine rotational speed,the change gear ratio of the main transmission device 9 and the vehiclespeed becomes as follows. In the high rotational speed region NH, therelationship is obtained in which the vehicle speed is proportional tothe engine rotational speed so that the vehicle speed corresponds to theengine rotational speed at the corrected fastest change gear ratio (setchange gear ratio) of the main transmission device 9 corresponding tothe operation position of the main shift lever 20. In the mediumrotational speed region NM, the relationship is obtained in which thevehicle speed is proportional to the engine rotational speed and to thechange gear ratio of the main transmission device 9 so that, when theengine rotational speed reaches the first set rotational speed (idlingrotational speed) N1, the vehicle speed becomes the corrected very lowset speed based on the operation position of the main shift lever 20,and when the engine rotational speed reaches the second set rotationalspeed N2 at which the maximum torque is exerted, the vehicle speedcorresponds to the engine rotational speed at the corrected fastestchange gear ratio of the main transmission device 9 described above. Inthe low rotational speed region NL, the relationship is obtained inwhich the vehicle speed corresponds to the engine rotational speed atthe change gear ratio of the main transmission device 9 which isinversely proportional to the engine rotational speed so that thevehicle speed becomes constant at the corrected very low set speeddescribed above.

On the other hand, as shown in FIG. 10 (B), in the corrected secondvehicle speed setting data, the relationship among the engine rotationalspeed, the change gear ratio of the main transmission device 9 and thevehicle speed becomes as follows. In the high rotational speed regionNH, the relationship is obtained in which the vehicle speed isproportional to the engine rotational speed so that the vehicle speedcorresponds to the engine rotational speed at the corrected fastestchange gear ratio (set change gear ratio) of the main transmissiondevice 9 corresponding to the operation position of the main shift lever20. In the medium rotational speed region NM, the relationship isobtained in which the vehicle speed is proportional to the enginerotational speed and to the change gear ratio of the main transmissiondevice 9 so that, when the engine rotational speed reaches the first setrotational speed (idling rotational speed) N1, the vehicle speed becomesthe corrected low set speed based on the operation position of the mainshift lever 20, and when the engine rotational speed reaches the secondset rotational speed N2 which is slightly smaller than the ratedrotational speed, the vehicle speed corresponds to the engine rotationalspeed at the corrected fastest change gear ratio of the maintransmission device 9 described above. In the low rotational speedregion NL, the relationship is obtained in which the vehicle speedcorresponds to the engine rotational speed at the change gear ratio ofthe main transmission device 9 which is proportional to the enginerotational speed so that, when the engine rotational speed reaches thefirst set rotational speed (idling rotational speed) N1, the vehiclespeed becomes the corrected low set speed described above, and when theengine rotational speed is zero, the main transmission device 9 becomesneutral and the vehicle speed becomes zero.

Then, the control target vehicle speed is set based on the correctedvehicle speed setting data and the output of the engine sensor 62, sothat the relationship among the engine rotational speed, the change gearratio of the main transmission device 9, and the vehicle speed isestablished with the corrected vehicle speed setting data.

Hereinbelow, the vehicle speed setting control will be de described withreference to the flow chart of FIG. 11. The speed change step of theauxiliary transmission device 10 is detected from the output of thelever sensor 60 for auxiliary shift lever (step #1101). When the speedchange step is the low-speed step for low-speed working or thehigh-speed step for high-speed working, the first vehicle speed settingdata for work traveling is adopted as the vehicle speed setting data(step #1102), and when the speed change step is the fastest step forhigh-speed moving, the second vehicle speed setting data for high-speedmoving is adopted as the vehicle speed setting data (step #1103). Next,a gain corresponding to the vehicle speed setting data is read out fromthe output of the lever sensor 59 for main shift lever (steps #1104,#1105), and the vehicle speed setting data is corrected by multiplyingthe adopted vehicle speed setting data by the gain, to therebyautomatically correct the vehicle speed setting data to an appropriateone for which the set change gear ratio (the fastest change gear ratioof the main transmission device 9 set by the main shift lever 20)corresponding to the operation position of the main shift lever 20 isused as criterion (steps #1106, #1107). Then, the engine rotationalspeed from the output of the engine sensor 62 is read (steps #1108,#1109), and the control target vehicle speed for the output of thevehicle speed sensor 63 is determined based on the corrected vehiclespeed setting data and the read engine rotational speed (steps #1110,#1111).

Then, the control target vehicle speed determined by the vehicle speedsetting control is set either for forward traveling or for reversetraveling based on the output of the lever sensor 61 for FR lever, theactuation of the change gear ratio altering unit C is controlled in sucha manner that the output of the vehicle speed sensor 63 matches the setcontrol target vehicle speed for forward traveling or for reversetraveling (the output of the vehicle speed sensor 63 falls on the deadband of the control target vehicle speed for forward traveling or forreverse traveling), and the operation angles of the pump swash plate 31Band the motor swash plate 32B are altered, to thereby perform thevehicle speed feedback control to alter the change gear ratio of themain transmission device 9.

With this configuration, for example, when the main shift lever 20 isoperated from a zero-speed position to the desired operation position,and at the same time, when an accelerator operation is performed by theaccelerator lever 17 or the accelerator pedal 18 with the predetermineddriving condition being established, while the engine rotational speedis in the medium rotational speed region NM, the engine rotational speedis increased or decreased in accordance with the accelerator operationand the change gear ratio of the main transmission device 9 is alteredin proportion to this increased or decreased engine rotational speed, insuch a manner that the vehicle speed is changed at a constant rate ofchange which is larger than the rate of change in the high rotationalspeed region NH.

With this configuration, in the case where the engine rotational speedis gradually increased from the idling rotational speed by theaccelerator operation and the vehicle body is started, as the enginerotational speed becomes closer to the idling rotational speed, thechange gear ratio of the main transmission device 9 becomes a largerchange gear ratio on the low-speed side, and thus a load on the engine 3is reduced. Therefore, as compared with the case where the vehicle bodyis started in a state in which the change gear ratio of the maintransmission device 9 is retained to the set change gear ratiocorresponding to the operation position of the main shift lever 20, adecrease in the engine rotational speed and generation of engine stallcaused by overload upon starting can be effectively suppressed, and thestarting of the vehicle body can be smoothly performed.

In the case of a trans-ridge traveling in which the vehicle body entersor leaves an agricultural field, a loading and unloading traveling inwhich the vehicle body is mounted or dismounted on or from a loadingspace of a truck using a running board, or the like, as the enginerotational speed is made closer to the idling rotational speed by theaccelerator operation, the change gear ratio of the main transmissiondevice 9 becomes a larger change gear ratio on the low-speed side and aload on the engine 3 is reduced, and the vehicle speed is decreased to alarge extent. Therefore, the trans-ridge traveling, the loading andunloading traveling or the like can be excellently performed atlow-speed. Especially, by operating the auxiliary shift lever 21 to thelow-speed position or the high-speed position and adopting the firstvehicle speed setting data, the speed change step of the auxiliarytransmission device 10 can be set to the low-speed step or thehigh-speed step with a high torque and the vehicle speed can be set to avery low speed. Therefore, the trans-ridge traveling, the loading andunloading traveling or the like can be more excellently performed atvery low speed with a high torque.

Furthermore, in the medium rotational speed region NM, as compared withthe configuration in which the engine rotational speed is simplyincreased or decreased in accordance with the accelerator operation, anadjustment range of the vehicle speed by the accelerator operation iswider, and the amount of change of the vehicle speed relative to an theaccelerator operation amount is larger. Therefore, the acceleration anddeceleration by the accelerator operation is facilitated and themovement utilizing the medium rotational speed region NM can becomfortably performed. Especially, in a high-speed moving state in whichthe auxiliary shift lever 21 is operated to the high-speed movementposition, by adopting the second vehicle speed setting data in which thesecond set rotational speed N2 is set to a rotational speed slightlylower than the rated rotational speed of the engine 3, in nearly wholerange from the idling rotational speed to the rated rotational speed,the acceleration and deceleration by the accelerator operation isfacilitated and the high-speed moving can be comfortably performed.

On the other hand, while the engine rotational speed is in the highrotational speed region NH, even though the engine rotational speedchanges due to the accelerator operation, traveling load or workingload, the change gear ratio of the main transmission device 9 becomesconstant at the set change gear ratio corresponding to the operationposition of the main shift lever 20, and therefore, the relationship inwhich the vehicle speed is proportional to the engine rotational speedcan be retained. Especially, in a work traveling state in which theauxiliary shift lever 21 is operated to the low-speed position or thehigh-speed position, by adopting the first vehicle speed setting data inwhich the rotational speed at which the engine 3 exerts the maximumtorque is set to the second set rotational speed N2, the high rotationalspeed region NH spans a wide range from the second set rotational speedN2 at which the engine 3 exerts the maximum torque to the ratedrotational speed, and in this wide high rotational speed region NH, therelationship in which the vehicle speed is proportional to the enginerotational speed can be retained. Accordingly, when the working device,such as rotary tilling device, configured to be actuated by non-changedpower (non-changed rotational speed) from the engine 3 taken out fromthe PTO shaft 16, is connected to a rear portion of the vehicle body inorder to perform work, by operating the auxiliary shift lever 21 to thelow-speed position or the high-speed position, a relationship in whicheach of the vehicle speed and the actuation speed of the working deviceis proportional to the engine rotational speed can be retained in thewide high rotational speed region NH spanning from the second setrotational speed N2 at which the engine 3 exerts the maximum torque tothe rated rotational speed. As a result, the disturbance in the workingtrail of the working device caused by inability to retain the constantrelationship between the vehicle speed and the actuation speed of theworking device can be prevented, and the working trail of the workingdevice can be finished in an excellent state in which the work isuniformly performed. Especially, when the rotary tilling device isconnected to the rear portion of the vehicle body in order to performthe tillage work, the tilling trail of the rotary tilling device can befinished in an excellent tilling state in which the tilling is uniformlyperformed.

In the case where the engine rotational speed is reduced from the highrotational speed region NH to the medium rotational speed region NM dueto the traveling load, working load or the like, as the enginerotational speed is decreased in the medium rotational speed region NM,the change gear ratio of the main transmission device 9 becomes a largerchange gear ratio on the low-speed side, and thus a load on the engine 3is reduced. Therefore, the engine 3 can be imparted with a viscousproperty, to thereby effectively suppress a decrease in the enginerotational speed and generation of engine stall which may otherwise becaused by overload. In addition, with this configuration, in the casewhere the vehicle body is started by a press canceling operation of theright and left brake pedals 19 or the stop pedal 23, or by a swingingoperation of the FR lever 22 from the neutral position to the forwardposition or the reverse position, performed while the engine rotationalspeed is retained in the high rotational speed region NH, even thoughthe engine rotational speed is decreased due to the traveling load orworking load at that moment, if the engine rotational speed is decreasedas low as the medium rotational speed region NM, the engine 3 can beimparted with a viscous property and the decrease in the enginerotational speed and generation of engine stall caused by overload canbe effectively suppressed. Therefore, the starting of the vehicle bodyutilizing the right and left brake pedals 19, the stop pedal 23, or theFR lever 22, can be smoothly performed.

For the engine rotational speed which is decreased to the low rotationalspeed region NL due to the traveling load, working load or the like, thesetting is made in such a manner that, with the first vehicle speedsetting data to be adopted upon work traveling, the vehicle speedbecomes constant at a very low set speed, and with the second vehiclespeed setting data to be adopted upon high-speed moving, the vehiclespeed becomes zero when the engine rotational speed becomes zero.Therefore, during a time period in which the engine rotational speed isreduced to zero, the main transmission device 9 never becomes neutral,and with this configuration, even though the engine 3 is operated aphenomenon in which a driver senses a discomfort can be prevented, suchas stopping traveling of the vehicle body caused by the maintransmission device 9 becoming neutral, due to a reduction in the enginerotational speed.

Moreover, in the low rotational speed region NL, the traveling state ata low speed or very low speed is retained, and thus in the lowrotational speed region NL, the engine 3 can be imparted with a viscousproperty to some degree. With this viscosity, the driver is allowed torecognize overload on the engine 3 in the low rotational speed regionNL, to thereby motivate the driver to take some measures, such asdeceleration operation of the main shift lever 20 to reduce the load. Ifno measure is taken, the engine stall is generated due to the overload,and thus the driver is allowed to re-recognize the overload on theengine 3 in the low rotational speed region NL.

It should be noted that, instead of the configuration described above,the vehicle speed control may be configured in such a manner that acombination of the change gear ratio of the main transmission device 9and a change gear ratio of the auxiliary transmission device 10 is usedas change gear ratio of a transmission device for traveling, that datashowing a relationship among the engine rotational speed, the changegear ratio of the transmission device for traveling, and the vehiclespeed (an output rotational speed of the auxiliary transmission device10) is adopted as the vehicle speed setting data, and that a vehiclespeed sensor for detecting the output rotational speed of the auxiliarytransmission device 10 as a vehicle speed upon control is provided.Alternatively, the vehicle speed control may be configured in such amanner that a combination of the change gear ratio of the maintransmission device 9, the change gear ratio of the auxiliarytransmission device 10, and a final reduction ratio by the differentialdevice 13 for rear wheel or the like is used as a change gear ratio of arear wheel transmission system, and that data showing a relationshipamong the engine rotational speed, the change gear ratio of the rearwheel transmission system and the vehicle speed is adopted as thevehicle speed setting data.

Referring to FIGS. 4A, 4B, 4C, 12 and 13, the vehicle speed control unit27B includes: a change gear ratio determining unit 27Ba configured todetermine the change gear ratio of the main transmission device 9 whichis set based on the operation position of the main shift lever 20, theengine rotational speed and the like; a decrease detection unit 27Bbconfigured to detect a decrease amount of the engine rotational speed(hereinbelow, referred to as “engine drop amount”) from the setrotational speed; a first switching control unit 27Bc configured toswitch the speed change step of the variable capacity motor 32 based onthe output of the lever sensor 59 for main shift lever; a secondswitching control unit 27Bd configured to switch the speed change stepof the variable capacity motor 32 based on the determination of thechange gear ratio determining unit 27Ba; and a third switching controlunit 27Be configured to switch the speed change step of the variablecapacity motor 32 based on an output of the decrease detection unit27Bb.

Then, the vehicle speed control unit 27B is configured to switch acontrol actuation regarding the switching of the speed change step ofthe variable capacity motor 32, in accordance with the speed change stepof the auxiliary transmission device 10 read out from the output of thelever sensor 60 for auxiliary shift lever. Specifically, when the speedchange step of the auxiliary transmission device 10 is the low-speedstep for low-speed working, the vehicle speed control unit 27B actuatesthe first switching control unit 27Bc; when the speed change step of theauxiliary transmission device 10 is the high-speed step for high-speedworking, the vehicle speed control unit 27B actuates the first switchingcontrol unit 27Bc and the third switching control unit 27Be; and whenthe speed change step of the auxiliary transmission device 10 is thefastest step for high-speed moving, the vehicle speed control unit 27Bactuates the second switching control unit 27Bd and the third switchingcontrol unit 27Be.

It should be noted that, the change gear ratio determining unit 27Ba isconfigured to determine whether or not the control target vehicle speedwhich is set based on the adopted corrected vehicle speed setting data(the vehicle speed setting data which is corrected based on theoperation position of the main shift lever 20) and the output of theengine sensor 62 exceeds the limit value of the speed that can beattained at the low-speed step of the variable capacity motor 32, tothereby determine whether or not the change gear ratio of the maintransmission device 9 which is set based on the operation position ofthe main shift lever 20, the engine rotational speed and the like is achange gear ratio that can be attained at the low-speed step of thevariable capacity motor 32.

The decrease detection unit 27Bb is configured to read out the setrotational speed of the engine 3 which is set by the accelerator lever17 or the accelerator pedal 18, based on the output of the lever sensor30 for speed governing lever, and to compute the engine drop amountbased on the set rotational speed of the engine 3 read out and theoutput of the engine sensor 62.

Hereinbelow, a first switching control in which the speed change step ofthe variable capacity motor 32 is switched by a control actuation of thefirst switching control unit 27Bc will be described with reference tothe flow chart of FIG. 12. The operation position of the main shiftlever 20 is determined based on the output of the lever sensor 59 formain shift lever (steps #1201, #1202). When the operation position ofthe main shift lever 20 is within a low-speed set region including thezero-speed position, the disconnection of the electrical continuity tothe speed change operation valve 43 is continued so that the speedchange step of the variable capacity motor 32 is kept as the low-speedstep (step #1203). When the operation position of the main shift lever20 is in a high-speed set region including the fastest position, theelectrical continuity to the speed change operation valve 43 iscontinued so that the speed change step of the variable capacity motor32 is kept as the high-speed step (step #1204). When the operationposition of the main shift lever 20 is in the medium-speed set regionbetween the low-speed set region and the high-speed set region, anoperation process to the medium-speed set region is detected (step#1205), and when the main shift lever 20 is positioned in themedium-speed set region after the operation from the low-speed setregion, the speed change step of the variable capacity motor 32 isretained at the low-speed step, and when positioned in the medium-speedset region after the operation from the high-speed set region, theelectrical continuity to the speed change operation valve 43 iscontrolled so that the speed change step of the variable capacity motor32 is kept as the high-speed step (steps #1203, #1204). In the casewhere the speed change step of the variable capacity motor 32 isretained at the low-speed step, it is determined whether or not anoperation of the main shift lever 20 from the medium-speed set region tothe high-speed set region is performed, based on the output of the leversensor 59 for main shift lever (step #1206), and when the operation tothe high-speed set region is detected, a current supply to the speedchange operation valve 43 is started so that the speed change step ofthe variable capacity motor 32 is switched from the low-speed step tothe high-speed step (step #1207), and then the electrical continuity tothe variable speed valve 38 for forward traveling or the variable speedvalve 39 for reverse traveling is controlled in such a manner that adeceleration operation of the pump swash plate 31B of the variablecapacity pump 31 is performed with an operation amount which is set soas to compensate (offset) an acceleration amount of the change gearratio of the main transmission device 9 changed along with switching ofthe variable capacity motor 32 from the low-speed step to the high-speedstep (step #1208). In the case where the speed change step of thevariable capacity motor 32 is retained at the high-speed step, it isdetermined whether or not the operation of the main shift lever 20 fromthe medium-speed set region to the low-speed set region is performed,based on the output of the lever sensor 59 for main shift lever (step#1209), and when the operation to the low-speed set region is detected,the electrical continuity to the speed change operation valve 43 isdisconnected so that the speed change step of the variable capacitymotor 32 is switched from the high-speed step to the low-speed step(step #1210), and then the electrical continuity to the variable speedvalve 38 for forward traveling or the variable speed valve 39 forreverse traveling is controlled in such a manner that an accelerationoperation of the pump swash plate 31B of the variable capacity pump 31is performed with an operation amount which is set so as to compensate(offset) a deceleration amount of the change gear ratio of the maintransmission device 9 changed along with switching of the variablecapacity motor 32 from the high-speed step to the low-speed step (step#1211).

It should be noted that the medium-speed set region of the main shiftlever 20 is a region in which the main shift lever 20 can set the changegear ratio of the main transmission device 9 that can be attained byswitching the speed change step of the variable capacity motor 32 toeither the low-speed step or the high-speed step.

With this configuration, when the main shift lever 20 is positioned inthe low-speed set region, or when positioned in the medium-speed setregion after the operation from the low-speed set region, and at thesame time, the above-described predetermined driving condition isestablished, the vehicle speed control unit 27B controls the electricalcontinuity to the variable speed valve 38 for forward traveling or thevariable speed valve 39 for reverse traveling to perform a speed changeoperation of the pump swash plate 31B of the variable capacity pump 31so that the control target vehicle speed which is set based on theadopted corrected vehicle speed setting data and the output of theengine sensor 62 is attained while retaining the speed change step ofthe variable capacity motor 32 to the low-speed step.

In addition, when the main shift lever 20 is positioned in thehigh-speed set region, or when positioned in the medium-speed set regionafter the operation from the high-speed set region, in the case wherethe above-described predetermined driving condition is established, thevehicle speed control unit 27B controls the electrical continuity to thevariable speed valve 38 for forward traveling or the variable speedvalve 39 for reverse traveling to perform the speed change operation ofthe pump swash plate 31B of the variable capacity pump 31 so that thecontrol target vehicle speed which is set based on the adopted correctedvehicle speed setting data and the output of the engine sensor 62 isattained while retaining the speed change step of the variable capacitymotor 32 to the high-speed step.

Then, in the case where an acceleration operation of the main shiftlever 20 positioned in the low-speed set region or the medium-speed setregion after the operation from the low-speed set region is performed tothe high-speed set region while the above-described predetermineddriving condition is established, the electrical continuity to thevariable speed valve 38 for forward traveling or the variable speedvalve 39 for reverse traveling is controlled so that the accelerationoperation of the pump swash plate 31B of the variable capacity pump 31is performed in accordance with the acceleration operation of the mainshift lever 20 until the main shift lever 20 reaches the high-speed setregion. When the main shift lever 20 reaches the high-speed set region,by the control actuation of the first switching control unit 27Bc asdescribed above, a current supply to the speed change operation valve 43is started so that the speed change step of the variable capacity motor32 is switched from the low-speed step to the high-speed step, and thenthe electrical continuity to the variable speed valve 38 for forwardtraveling or the variable speed valve 39 for reverse traveling iscontrolled in such a manner that the deceleration operation of the pumpswash plate 31B of the variable capacity pump 31 is performed with anoperation amount which is set so as to compensate the accelerationamount of the change gear ratio of the main transmission device 9changed along with switching of the variable capacity motor 32 from thelow-speed step to the high-speed step.

To the contrary, in the case where a deceleration operation of the mainshift lever 20 positioned in the high-speed set region or themedium-speed set region after the operation from the high-speed setregion is performed to the low-speed set region while theabove-described predetermined driving condition is established, theelectrical continuity to the variable speed valve 38 for forwardtraveling or the variable speed valve 39 for reverse traveling iscontrolled so that the deceleration operation of the pump swash plate31B of the variable capacity pump 31 is performed in accordance with thedeceleration operation of the main shift lever 20 until the main shiftlever 20 reaches the low-speed set region. When the main shift lever 20reaches the low-speed set region, by the control actuation of the firstswitching control unit 27Bc as described above, the electricalcontinuity to the speed change operation valve 43 is disconnected sothat the speed change step of the variable capacity motor 32 is switchedfrom the high-speed step to the low-speed step, and then the electricalcontinuity to the variable speed valve 38 for forward traveling or thevariable speed valve 39 for reverse traveling is controlled in such amanner that the acceleration operation of the pump swash plate 31B ofthe variable capacity pump 31 is performed with an operation amountwhich is set so as to compensate the deceleration amount of the changegear ratio of the main transmission device 9 changed along withswitching of the variable capacity motor 32 from the high-speed step tothe low-speed step.

Hereinbelow, a second switching control in which the speed change stepof the variable capacity motor 32 is switched by a control actuation ofthe second switching control unit 27Bd will be described with referenceto the flow chart of FIG. 13. Based on the determination by the changegear ratio determining unit 27Ba, it is determined whether or not thecontrol target vehicle speed which is determined based on the adoptedcorrected vehicle speed setting data and the output of the engine sensor62 exceeds the limit value of the speed that can be attained at thelow-speed step of the variable capacity motor 32 (step #1301). When thecontrol target vehicle speed does not exceed the limit value, it isdetected whether or not the control target vehicle speed exceeds thelimit value of the speed (step #1302), and when the control targetvehicle speed does not exceed the limit value of the speed, thedisconnection of the electrical continuity to the speed change operationvalve 43 is continued so that the speed change step of the variablecapacity motor 32 is kept as the low-speed step (step #1303). When thecontrol target vehicle speed exceeds the limit value, a current supplyto the speed change operation valve 43 is started so that the speedchange step of the variable capacity motor 32 is switched from thelow-speed step to the high-speed step (step #1304), and then theelectrical continuity to the variable speed valve 38 for forwardtraveling or the variable speed valve 39 for reverse traveling iscontrolled in such a manner that a deceleration operation of the pumpswash plate 31B of the variable capacity pump 31 is performed with anoperation amount which is set so as to compensate (offset) theacceleration amount of the change gear ratio of the main transmissiondevice 9 changed along with switching of the variable capacity motor 32from the low-speed step to the high-speed step (step #1305). When thecontrol target vehicle speed exceeds the limit value of the speed in thestep #1301, it is detected whether or not the control target vehiclespeed is reduced to or below the limit value of the speed (step #1306),and when the control target vehicle speed is not reduced to or below thelimit value of the speed, the electrical continuity to the speed changeoperation valve 43 is continued so that the speed change step of thevariable capacity motor 32 is kept as the high-speed step (step #1307).When the control target vehicle speed is reduced to or below the limitvalue, the electrical continuity to the speed change operation valve 43is disconnected so that the speed change step of the variable capacitymotor 32 is switched from, the high-speed step to the low-speed step(step #1308), and then the electrical continuity to the variable speedvalve 38 for forward traveling or the variable speed valve 39 forreverse traveling is controlled in such a manner that the accelerationoperation of the pump swash plate 31B of the variable capacity pump 31is performed with an operation amount which is set so as to compensate(offset) the deceleration amount of the change gear ratio of the maintransmission device 9 changed along with switching of the variablecapacity motor 32 from the high-speed step to the low-speed step (step#1309).

It should be noted that the speed that can be attained at the low-speedstep of the variable capacity motor 32 is defined as a speed obtainedbased on: the change gear ratio obtained by, while the speed change stepof the variable capacity motor 32 is set to the low-speed step, alteringthe operation angle of the pump swash plate 31B of the variable capacitypump 31 from zero to the operation limitation angle; and the enginerotational speed from the second vehicle speed setting data,corresponding to this change gear ratio. The limit value of this speedis defined as a speed obtained based on: the fastest change gear ratioat the low-speed step obtained by, while the speed change step of thevariable capacity motor 32 is set to the low-speed step, altering theoperation angle of the pump swash plate 31B of the variable capacitypump 31 to the operation limitation angle; and the engine rotationalspeed from the second vehicle speed setting data, corresponding to thisfastest change gear ratio.

With this configuration, for example, when the main shift lever 20 ispositioned at the fastest position, and at the same time, theabove-described predetermined driving condition is established, in thecase where a pressing operation of the accelerator pedal 18 isperformed, the vehicle speed control unit 27B controls the electricalcontinuity to the variable speed valve 38 for forward traveling or thevariable speed valve 39 for reverse traveling to perform the speedchange operation of the pump swash plate 31B of the variable capacitypump 31 so that the control target vehicle speed is obtained with thespeed change step of the variable capacity motor 32 being retained tothe low-speed step, until the control target vehicle speed which is setbased on the adopted corrected vehicle speed setting data and the outputof the engine sensor 62 changeable in accordance with the pressingoperation of the accelerator pedal 18 exceeds the above-described limitvalue.

When the control target vehicle speed exceeds the above-described limitvalue, along with it, by the control actuation of the second switchingcontrol unit 27Bd as described above, the current supply to the speedchange operation valve 43 is started so that the speed change step ofthe variable capacity motor 32 is switched from the low-speed step tothe high-speed step, and then the electrical continuity to the variablespeed valve 38 for forward traveling or the variable speed valve 39 forreverse traveling is controlled in such a manner that a decelerationoperation of the pump swash plate 31B of the variable capacity pump 31is performed with an operation amount which is set so as to compensatethe acceleration amount of the change gear ratio of the maintransmission device 9 changed along with switching of the variablecapacity motor 32 from the low-speed step to the high-speed step. Whilethe control target vehicle speed is kept above the limit value, theelectrical continuity to the variable speed valve 38 for forwardtraveling or the variable speed valve 39 for reverse traveling iscontrolled to perform the speed change operation of the pump swash plate31B of the variable capacity pump 31 so that the control target vehiclespeed is obtained with the speed change step of the variable capacitymotor 32 being retained to the high-speed step.

Afterward, when the control target vehicle speed is reduced to theabove-described limit value by the operation of the accelerator pedal 18in a press canceling direction, along with it, by the control actuationof the second switching control unit 27Bd as described above, theelectrical continuity to the speed change operation valve 43 isdisconnected so that the speed change step of the variable capacitymotor 32 is switched from the high-speed step to the low-speed step, andthen the electrical continuity to the variable speed valve 38 forforward traveling or the variable speed valve 39 for reverse travelingis controlled in such a manner that the acceleration operation of thepump swash plate 31B of the variable capacity pump 31 is performed withan operation amount which is set so as to compensate the decelerationamount of the change gear ratio of the main transmission device 9changed along with switching of the variable capacity motor 32 from thehigh-speed step to the low-speed step. While the control target vehiclespeed is kept below the limit value, the electrical continuity to thevariable speed valve 38 for forward traveling or the variable speedvalve 39 for reverse traveling is controlled to perform the speed changeoperation of the pump swash plate 31B of the variable capacity pump 31so that the control target vehicle speed is obtained with the speedchange step of the variable capacity motor 32 being retained to thelow-speed step.

Hereinbelow, a third switching control in which the speed change step ofthe variable capacity motor 32 is switched by a control actuation of thethird switching control unit 27Be will be described with reference tothe flow chart of FIG. 14. The speed change step of the variablecapacity motor 32 is determined based on an electrical continuity stateof the speed change operation valve 43 (step #1401). When the speedchange step is the high-speed step, the engine drop amount is monitoredbased on the output of the decrease detection unit 27Bb (step #1402),and it is determined whether or not the engine drop amount reaches a setamount (for example a value which is 20% of the set rotational speed)(step #1403). When the engine drop amount reaches the set amount, theelectrical continuity to the speed change operation valve 43 isdisconnected so that the speed change step of the variable capacitymotor 32 is switched from the high-speed step to the low-speed step(step #1404). Then, it is determined whether or not the engine dropamount is recovered to a predetermined acceptable range (step #1405),and when it is recovered to the predetermined acceptable range, acurrent supply to the speed change operation valve 43 is started so thatthe speed change step of the variable capacity motor 32 is switched fromthe low-speed step to the high-speed step (step #1406).

With the configuration described above, in a low-speed working state inwhich the speed change step of the auxiliary transmission device 10 isset to the low-speed step, and in a high-speed working state in whichthe speed change step of the auxiliary transmission device 10 is set tothe high-speed step, when it is desired to secure a higher torque inorder to more comfortably perform heavy loading work or the like, bypositioning the main shift lever 20 at the low-speed set region orshifting the main shaft lever 20 from the low-speed set region to themedium-speed set region, the speed change step of the variable capacitymotor 32 can be set to the low-speed step, and a higher torque can besecured. Accordingly, the starting, traveling or the like of the vehiclebody in heavy loading work or the like can be smoothly performed, andheavy loading work can be more comfortably performed.

Then, in the low-speed working state in which the speed change step ofthe auxiliary transmission device 10 is set to the low-speed step, sincea high torque is secured by the speed change step, a decrease in theengine rotational speed and generation of engine stall caused byoverload are unlikely to occur, and thus high-low switching of thevariable capacity motor 32 in the main transmission device 9 isperformed solely through the operation of the main shift lever 20 bydriver's will. Accordingly, even when the variable capacity motor 32 ofthe main transmission device 9 is switched to the high-speed step forthe purpose of enhancing the work efficiency in the low-speed workingstate, the speed change step of the variable capacity motor 32 is neverunexpectedly switched from the high-speed step to the low-speed stepwhich may otherwise be caused by the reduction in the engine rotationalspeed, and as long as the engine rotational speed is within the highrotational speed region NH, the change gear ratio of the maintransmission device 9 set by the operation of the main shift lever 20,the accelerator lever 17 or the accelerator pedal 18 based on the willof the driver who pays attention to the work can be retained. When thereduction in the engine rotational speed is notable, the variablecapacity motor 32 of the main transmission device 9 can be switched tothe low-speed step by the operation of the main shift lever 20 by thedriver's will, to thereby suppress or prevent the reduction in theengine rotational speed. As a result, work, such as rotary tillage work,can be excellently performed in which the vehicle speed and theactuation speed of the working device are desired to be retained in arelationship in which they are proportional to the engine rotationalspeed.

In addition, in the high-speed working state in which the speed changestep of the auxiliary transmission device 10 is set to the high-speedstep, though a relatively high torque is secured by the speed changestep, a decrease in the engine rotational speed and generation of enginestall caused by overload are likely to occur as compared with the caseof the low-speed step. Therefore, the high-low switching of the variablecapacity motor 32 in the main transmission device 9 is configured notonly to be performed solely through the operation of the main shiftlever 20 by the driver's will, but also to be performed automaticallybased on the engine drop amount. With this configuration, while thehigh-low switching of the variable capacity motor 32 in the maintransmission device 9 can be performed by the driver's will inaccordance with the work, the generation of engine stall caused byoverload can be prevented.

Furthermore, in both of the low-speed working state and the high-speedworking state, during low-speed traveling in which the change gear ratioof the main transmission device 9 is set to the large change gear ratioon the low-speed side, the speed change step of the variable capacitymotor 32 becomes the low-speed step. Accordingly, for the operationangle of the pump swash plate 31B of the variable capacity pump 31, anangle on the high-speed side exhibiting high hydraulic pressuretransmission efficiency can be adopted, and as a result, the vehiclespeed during the low-speed traveling can be stabilized.

Furthermore, in the medium-speed set region, hysteresis is imparted, andthus a frequent high-low switching of the variable capacity motor 32 bythe operation of the main shift lever 20 can be prevented.

On the other hand, in the high-speed moving state in which the speedchange step of the auxiliary transmission device 10 is set to thefastest step, a torque is low due to the speed change step, andtherefore, as long as the control target vehicle speed is a speed thatcan be attained at the low-speed step of the variable capacity motor 32,the speed change step of the variable capacity motor 32 is retained tothe low-speed step to secure a high torque. With this configuration,during the starting or traveling of the vehicle body in the high-speedmoving state in which the speed change step of the auxiliarytransmission device 10 is set to the fastest step, a decrease in theengine rotational speed and generation of engine stall caused byoverload can be effectively suppressed. In addition, by retaining thevariable capacity motor 32 to the low-speed step as long as possible, anangle on the high-speed side exhibiting high hydraulic pressuretransmission efficiency can be used as the operation angle of the pumpswash plate 31B in a wider speed change region. Accordingly, hydraulicpressure transmission efficiency in the main transmission device 9 canbe enhanced during the starting or traveling in the high-speed movingstate in which the speed change step of the auxiliary transmissiondevice 10 is set to the fastest step, and a decrease in the enginerotational speed and generation of engine stall caused by overload canbe more effectively suppressed during the starting or traveling of thevehicle body in the high-speed moving state.

Furthermore, when the speed change step of the variable capacity motor32 is switched by the control actuation of the first switching controlunit 27Bc or the second switching control unit 27Bd, the pump swashplate 31B is operated using the operation amount which is set so as tocompensate an amount of change in the change gear ratio of the maintransmission device 9 changeable along with the switching, andtherefore, the speed change operation for altering the change gear ratioof the main transmission device 9 by the operation of the main shiftlever 20, the accelerator lever 17 or the accelerator pedal 18 can besmoothly performed non-stepwise, even though the switching of the speedchange step of the variable capacity motor 32 is performed.

Then, with respect to the low-speed working state in which the speedchange step of the auxiliary transmission device 10 is set to thelow-speed step, the high-speed working state in which the speed changestep is set to the high-speed step, and the high-speed moving state inwhich the speed change step is set to the fastest step, the controlactuation regarding the switching of the speed change step of thevariable capacity motor 32 suitable for each state can be automaticallyselected together with the switching of the speed change step of theauxiliary transmission device 10 by the auxiliary shift lever 21.

It should be noted that the medium-speed set region of the main shiftlever 20 is a region in which the main shift lever 20 can set the changegear ratio of the main transmission device 9 that can be attained byswitching the speed change step of the variable capacity motor 32 toeither the low-speed step or the high-speed step. It should be alsonoted that the speed that can be attained at the low-speed step of thevariable capacity motor 32 is defined as a speed obtained based on: thechange gear ratio obtained by, while the speed change step of thevariable capacity motor 32 is set to the low-speed step, altering theoperation angle of the pump swash plate 31B of the variable capacitypump 31 from zero to the operation limitation angle; and the enginerotational speed from the second vehicle speed setting data,corresponding to this change gear ratio. The limit value of this speedis defined as a speed obtained based on: the fastest change gear ratioat the low-speed step obtained by, while the speed change step of thevariable capacity motor 32 is set to the low-speed step, altering theoperation angle of the pump swash plate 31B of the variable capacitypump 31 to the operation limitation angle; and the engine rotationalspeed from the second vehicle speed setting data, corresponding to thefastest change gear ratio.

It should be noted that, the control actuation of the first switchingcontrol unit 27Bc may be configured in the following manner. When thefirst switching control unit 27Bc detects the operation of the mainshift lever 20 from the low-speed set region to the medium-speed setregion based on the output of the lever sensor 59 for main shift lever,a current supply to the speed change operation valve 43 is started sothat the speed change step of the variable capacity motor 32 is switchedfrom the low-speed step to the high-speed step, and then the electricalcontinuity to the variable speed valve 38 for forward traveling or thevariable speed valve 39 for reverse traveling is controlled in such amanner that a deceleration operation of the pump swash plate 31B of thevariable capacity pump 31 is performed with the operation amount whichis set so as to compensate the acceleration amount of the change gearratio of the main transmission device 9 changed along with switching ofthe variable capacity motor 32 from the low-speed step to the high-speedstep. When the operation of the main shift lever 20 from themedium-speed set region to the high-speed set region is detected, theelectrical continuity to the speed change operation valve 43 iscontrolled so that the speed change step of the variable capacity motor32 is kept as the high-speed step. When the operation of the main shiftlever 20 from the high-speed set region to the medium-speed set regionis detected, the electrical continuity to the speed change operationvalve 43 is disconnected so that the speed change step of the variablecapacity motor 32 is switched from the high-speed step to the low-speedstep, and then the electrical continuity to the variable speed valve 38for forward traveling or the variable speed valve 39 for reversetraveling is controlled in such a manner that the acceleration operationof the pump swash plate 31B of the variable capacity pump 31 isperformed with the operation amount which is set so as to compensate thedecelerated amount of the change gear ratio of the main transmissiondevice 9 changed along with switching of the variable capacity motor 32from the high-speed step to the low-speed step. When the operation ofthe main shift lever 20 from the medium-speed set region to thelow-speed set region is detected, the electrical continuity to the speedchange operation valve 43 is disconnected so that the speed change stepof the variable capacity motor 32 is kept as the low-speed step.

Referring to FIGS. 4B, 4C, 15 and 16, the display panel 24 is providedwith a tachometer 66 configured to show the engine rotational speed, anda liquid crystal display device 67 configured to show the vehicle speedand the like. The liquid crystal display device 67 is provided with: avehicle speed display part 67A configured to display the vehicle speedwith characters; a main speed change step display part 67B configured todisplay the speed change step of the main transmission device 9 withcharacters; a motor speed change step display part 67C configured todisplay the speed change step of the variable capacity motor 32 withcharacters; an auxiliary speed change step display part 67D configuredto display the speed change step of the auxiliary transmission device 10with characters; a set vehicle speed display part 67E configured todisplay the arbitrarily set vehicle speed with characters; and the like.An actuation of the liquid crystal display device 67 is controlled by acontrol actuation of a display control unit 27E provided as controlprogram in the ECU 27.

The vehicle speed display part 67A is arranged in a lower row in amiddle height region on a right side in the liquid crystal displaydevice 67. The main speed change step display part 67B is arranged in anupper region on a left side in the liquid crystal display device 67 insuch a manner that it is away from the vehicle speed display part 67A.The motor speed change step display part 67C is arranged in an upper rowin the middle height region on the left side in the liquid crystaldisplay device 67 so as to be positioned between the vehicle speeddisplay part 67A and the main speed change step display part 67B interms of height. The auxiliary speed change step display part 67D isarranged leftward of the main speed change step display part 67B in theliquid crystal display device 67. The set vehicle speed display part 67Eis arranged in a lower region on the right side in the liquid crystaldisplay device 67 so as to be positioned below the vehicle speed displaypart 67A.

The display control unit 27E includes a vehicle speed computing unit27Ea configured to compute an actual vehicle speed (hereinbelow,referred to as “actual speed”) based on: the output rotational speed ofthe main transmission device 9 which is output by the vehicle speedsensor 63; a change gear ratio in the set speed change step of theauxiliary transmission device 10 which is read out from the output ofthe lever sensor 60 for auxiliary shift lever; and the final reductionratio by the differential device 13 for rear wheel or the like and acircumferential length of the rear wheel 2 which are fixed dataregarding the computation of the vehicle speed, and the actual speedoutput by the vehicle speed computing unit 27Ea is displayed on thevehicle speed display part 67A.

The display control unit 27E includes a rated vehicle speed computingunit 27Eb configured to compute a rated vehicle speed which is atheoretical vehicle speed obtained when the engine rotational speedreaches the rated rotational speed in a speed change state arbitrarilyset by the main shift lever 20 and the auxiliary shift lever 21, basedon: the rated rotational speed of the engine 3; the vehicle speedsetting data (set change gear ratio of the main transmission device 9)corrected by the control actuation of the vehicle speed control unit 27Bbased on the output of the lever sensor 59 for main shift lever; thechange gear ratio in the set speed change step of the auxiliarytransmission device 10; the final reduction ratio; and thecircumferential length of the rear wheel 2, and the rated vehicle speedoutput by the rated vehicle speed computing unit 27Eb is displayed onthe main speed change step display part 67B without unit, as the speedchange step of the main transmission device 9.

The display control unit 27E is configured to read out the speed changestep of the variable capacity motor 32 from the control actuation of thevehicle speed control unit 27B, and to display the speed change step onthe motor speed change step display part 67C. In addition, the displaycontrol unit 27E is configured to read out the speed change step of theauxiliary transmission device 10 from the output of the lever sensor 60for auxiliary shift lever, and to display the speed change step on theauxiliary speed change step display part 67D.

The display control unit 27E includes a set vehicle speed computing unit27Ec configured to compute a set vehicle speed which is a theoreticalvehicle speed obtained when the engine rotational speed reaches thearbitrarily set rotational speed in the speed change state arbitrarilyset by the main shift lever 20 and the auxiliary shift lever 21, basedon: the set rotational speed of the engine 3 which is read out from theoutput of the lever sensor 30 for speed governing lever; the changegear′ ratio of the main transmission device 9 determined by the setrotational speed and the corrected vehicle speed setting data adopted bythe vehicle speed control unit 27B; the change gear ratio in the setspeed change step of the auxiliary transmission device 10; the finalreduction ratio; and the circumferential length of the rear wheel 2, andthe set vehicle speed output by the set vehicle speed computing unit27Ec is displayed on the set vehicle speed display part 67E.

With respect to the display on each display part 67A-67E, specifically,when the computed vehicle speed is 10 km/h, an indication of “10.0 km”is displayed, and when 4.5 km/h, an indication of “4.5 km” is displayedon the vehicle speed display part 67A. When the computed rated vehiclespeed is 12 km/h, an indication of “12.0” is displayed, and when 6 km/h,an indication of “6.0” is displayed on the main speed change stepdisplay part 67B as the speed change step of the main transmissiondevice 9. When the speed change step of the variable capacity motor 32which is read out from the control actuation of the vehicle speedcontrol unit 27B is the low-speed step, an indication of “L” isdisplayed, and when the high-speed step, an indication of “H” isdisplayed on the motor speed change step display part 67C. When thespeed change step of the auxiliary transmission device 10 which is readout from the output of the lever sensor 60 for auxiliary shift lever isthe low-speed step for working, an indication of “low” is displayed,when the high-speed step for working, an indication of “high” isdisplayed, and when the fastest step for high-speed moving, andindication of “high-speed” is displayed on the auxiliary speed changestep display part 67D. When the computed set vehicle speed is 10 km/h,an indication of “10.0 km” is displayed, and when 4.5 km/h, anindication of “4.5 km” is displayed on the set vehicle speed displaypart 67E. Then, the display on the set vehicle speed display part 67Efunctions as an interrupt display that shows a set vehicle speed at thatmoment for a set time (e.g., 5 seconds) when the operation by which thevehicle speed is changed is performed.

It should be noted that the operation by which the vehicle speed ischanged (or the operation that changes the vehicle speed) herein meansthat an operation of the accelerator lever 17, an operation of theaccelerator pedal 18, the dual pressing operation of the right and leftbrake pedals 19, the operation of the main shift lever 20, an operationof the auxiliary shift lever 21, the operation of the FR lever 22, andan operation of the stop pedal 23.

With the configuration described above, when the main shift lever 20 isoperated to alter the set change gear ratio of the main transmissiondevice 9, the speed change step of the main transmission device 9displayed on the main speed change step display part 67B is changed in acontinuous manner in accordance with the operation position of the mainshift lever 20 and therefore, the setting of the change gear ratio ofthe main transmission device 9 by the main shift lever 20 isfacilitated. Since the speed change step of the main transmission device9 displayed on the main speed change step display part 67B is the ratedvehicle speed which matches the highest speed obtained when theaccelerator operation is maximum in the speed change state set by themain shift lever 20 and the auxiliary shift lever 21, and therefore, thevehicle speed setting in accordance with work condition is facilitatedwithout using a vehicle speed table which stores the highest speed foreach speed change state. In addition, since the speed change step of themain transmission device 9 displayed on the main speed change stepdisplay part 67B is the rated vehicle speed, when the speed change stepof the auxiliary transmission device 10 is altered, a display range inthe main speed change step display part 67B is automatically switched inaccordance with the altered speed change step of the auxiliarytransmission device 10. Therefore, the vehicle speed setting inaccordance with work condition, including the switching of the speedchange step of the auxiliary transmission device 10, is facilitated.Moreover, the main speed change step display part 67B and the auxiliaryspeed change step display part 67D are arranged side by side andtherefore, an association between the speed change step of the maintransmission device 9 and the speed change step of the auxiliarytransmission device 10 is easily understood. Furthermore, the main speedchange step display part 67B is arranged at a position away from thevehicle speed display part 67A, the motor speed change step display part67C is arranged between the vehicle speed display part 67A and the mainspeed change step display part 67B, and the main speed change stepdisplay part 67B is displayed without unit. Therefore, even though theactual speed displayed on the vehicle speed display part 67A becomesequal to the speed change step of the main transmission device 9displayed on the main speed change step display part 67B as a result ofmaximizing the accelerator operation, their indications can be easilydistinguished.

In addition, in the set vehicle speed display part 67E, when theoperation of the vehicle speed setting is performed by the acceleratorlever 17, the main shift lever 20 or the like, the set vehicle speed(reached vehicle speed predicted from the vehicle speed settingoperation) is instantly displayed, which is determined based on theoperation position of the accelerator lever 17 or the accelerator pedal18 and the operation positions of the main shift lever 20 and theauxiliary shift lever 21, and the displayed set vehicle speed is changedin a continuous manner in accordance with the vehicle speed settingoperation. Therefore, for example in a case where work is performed at adesired engine rotational speed and vehicle speed, the vehicle speedsetting can be simply and quickly performed regardless of the state ofthe vehicle including traveling and stopping. Then, since the display onthe set vehicle speed display part 67E is an interrupt display thatworks when the operation by which the vehicle speed is changed isperformed, the driver is allowed to clearly recognize the meaning of theindication in the set vehicle speed display part 67E. Furthermore,during the traveling at the set vehicle speed at which the vehicle speedto be displayed on the vehicle speed display part 67A and the setvehicle speed to be displayed on the set vehicle speed display part 67Ebecome the same, no indication is made on the set vehicle speed displaypart 67E, and therefore, a useless indication, such as the same values(speeds) are displayed one above the other in the liquid crystal displaydevice 67, can be prevented.

In addition, by displaying the speed change step of the variablecapacity motor 32 on the motor speed change step display part 67C, thedriver is allowed to easily recognize the speed change step of thevariable capacity motor 32 switched by the operation of the main shiftlever 20, the increase or decrease of the vehicle speed or the like.

Hereinbelow, preferred embodiments of the control system according tothe present invention will be listed.

(1) In a large load situation, by operating the speed change operationtool to the low-speed side, the swash plate angle of the variablecapacity motor can be changed to an angle on the low-speed side, and ahigh torque can be obtained. Accordingly, even in a large loadsituation, the work vehicle can smoothly travel. To the contrary, in asmall load situation, by operating the speed change operation tool tothe high-speed side, the swash plate angle of the variable capacitymotor can be changed to the angle on the high-speed side to obtain fastspeed, and therefore, the work efficiency can be enhanced.

In addition, during the low-speed traveling, by operating the speedchange operation tool to the low-speed side, the swash plate angle ofthe variable capacity motor is shifted to the angle on the low-speedside. Accordingly, the angle on the high-speed side exhibiting highhydraulic pressure transmission efficiency can be more frequently usedas the swash plate angle of the variable capacity pump, and thus thevehicle speed during the low-speed traveling can be stabilized, tothereby facilitate working at a low-speed.

(2) As long as the change gear ratio of the hydrostatic continuouslyvariable transmission device set based on the output of the operationposition detector is a change gear ratio that can be attained by theswash plate angle of the variable capacity motor on the low-speed side,the swash plate angle of the variable capacity motor is retained to theangle on the low-speed side. The low-speed side of the variable capacitymotor which can attain a high torque is utilized maximally, and theangle on the high-speed side exhibiting high hydraulic pressuretransmission efficiency can be more frequently used as the swash plateangle of the variable capacity pump.

As a result, while the traveling of the work vehicle in a large loadsituation can be smoothly performed, stability of the vehicle speed in awider low-speed region can be enhanced, and traveling and working in thewider low-speed region are facilitated.

(3) In a speed change state in which the swash plate angle of thevariable capacity motor is shifted to the angle on the high-speed sidewith a low torque, when the engine rotational speed is reduced to alarge extent so that the decrease amount of the engine rotational speedfrom the set rotational speed reaches the set amount, along with it, theswash plate angle of the variable capacity motor is shifted from theangle on the high-speed side to the angle on the low-speed side. Withthis configuration, the generation of engine stall caused by overloadcan be prevented.

(4) When the speed change step of the stepped transmission device is thelow-speed step for low-speed working that can secure a high torque, thefirst switching control unit is actuated. By operating the speed changeoperation tool to the low-speed side, the swash plate angle of thevariable capacity motor can be changed to the angle on the low-speedside, in accordance with the operation region on the low-speed side inwhich the speed change operation tool is located at that moment. To thecontrary, by operating the speed change operation tool to the high-speedside, the swash plate angle of the variable capacity motor can bechanged to the angle on the high-speed side, in accordance with theoperation region on the high-speed side in which the speed changeoperation tool is located at that moment.

With this configuration, for example in the case where heavy loadingwork is performed in which enough torque is not seemed to be obtainedonly by shifting the speed change step of the stepped transmissiondevice to the low-speed step for low-speed working, by operating thespeed change operation tool to the low-speed side, the swash plate angleof the variable capacity motor can be changed to the angle on thelow-speed side to thereby secure a higher torque, and therefore, even inheavy loading work or the like with a large load, the traveling of thework vehicle can be smoothly performed. To the contrary, in light loadwork or the like with a small load, by operating the speed changeoperation tool to the high-speed side, the swash plate angle of thevariable capacity motor can be changed to the angle on the high-speedside to obtain fast speed, and therefore, the work efficiency can beenhanced.

Then, the shifting of the swash plate angle of the variable capacitymotor by the actuation of the first switching control unit is performedsolely through the operation of the speed change operation tool by thedriver's will. Accordingly, even when the swash plate angle of thevariable capacity motor is shifted to the high-speed side, the swashplate angle of the variable capacity motor is never unexpectedly shiftedto the low-speed side which may otherwise be caused by the reduction inthe engine rotational speed, and the change gear ratio of thehydrostatic continuously variable transmission device suitable for thework set by the operation of the speed change operation tool by thedriver's will can be retained.

During the low-speed traveling, by operating the speed change operationtool to the low-speed side, the swash plate angle of the variablecapacity motor is shifted to the angle on the low-speed side.Accordingly, the angle on the high-speed side exhibiting high hydraulicpressure transmission efficiency can be more frequently used as theswash plate angle of the variable capacity pump, and thus the vehiclespeed during the low-speed traveling can be stabilized, to therebyfacilitate working at a low-speed.

In other words, by changing the swash plate angle of the variablecapacity motor based on the operation position of the speed changeoperation tool which is visually recognizable, advantages are obtainedin operability, and at the same time the variable capacity motor can beutilized more effectively, leading to enhancement in travelingperformance as well as workability.

When the speed change step of the stepped transmission device is thehigh-speed step for high-speed working that can secure a relatively hightorque, the first switching control unit and the third switching controlunit are actuated. Therefore, effects can be obtained which areapproximately the same as those obtained in the case where the low-speedstep for low-speed working is selected.

It should be noted that the high-speed step for high-speed workingexerts a lower torque than the low-speed step for low-speed working doesand thus engine stall caused by overload is likely to be generated.Therefore, the shifting of the swash plate angle of the variablecapacity motor is configured not only to be performed by the operationof the speed change operation tool by the driver's will, but also to beperformed automatically based on the decrease amount of the enginerotational speed. Accordingly, while it is configured to allow theoperation of the speed change operation tool to shift the swash plateangle of the variable capacity motor, in a speed change state in whichthe swash plate angle of the variable capacity motor is shifted to theangle on the high-speed side with a low torque, when the enginerotational speed is reduced to a large extent so that the decreaseamount of the engine rotational speed from the set rotational speedreaches the set amount, along with it, the swash plate angle of thevariable capacity motor is shifted from the angle on the high-speed sideto the angle on the low-speed side. With this configuration, thegeneration of engine stall caused by overload can be prevented.

When the speed change step of the stepped transmission device is thefastest step for high-speed moving with a low torque, the secondswitching control unit and the third switching control unit areactuated. Therefore, as long as the change gear ratio of the hydrostaticcontinuously variable transmission device set based on the output of theoperation position detector is a change gear ratio that can be attainedby the swash plate angle of the variable capacity motor on the low-speedside, the swash plate angle of the variable capacity motor is retainedto the angle on the low-speed side. Accordingly, the low-speed side ofthe variable capacity motor which can attain a high torque is utilizedmaximally, and the angle on the high-speed side exhibiting highhydraulic pressure transmission efficiency can be more frequently usedas the swash plate angle of the variable capacity pump.

As a result, while the moving of the work vehicle in a large loadsituation, such as ascending a slope, can be smoothly performed,stability of the vehicle speed in the wider low-speed region can beenhanced, and movability can be enhanced.

In addition, the fastest step for high-speed moving exerts further lowertorque than the high-speed step for high-speed working does and thusengine stall caused by overload is likely to be generated. Therefore,like in the case where the high-speed step for high-speed working isselected, the shifting of the swash plate angle of the variable capacitymotor is configured not only to be performed by the operation of thespeed change operation tool by the driver's will, but also to beperformed automatically based on the decrease amount of the enginerotational speed. With this configuration, the generation of enginestall caused by overload can be prevented.

Since the control actuation suitable for the speed change step of thestepped transmission device is automatically selected along with theselection of the speed change step of the stepped transmission device,the variable capacity motor can be utilized more appropriately andeffectively while enhancing the operability, leading to enhancement intraveling performance as well as workability.

(5) The second set rotational speed is set to the rotational speed atwhich the engine exerts the maximum torque.

According to this configuration, in a wide rotational speed region onthe high-speed side from the second set rotational speed at which theengine exerts the maximum torque to the rated rotational speed, thevehicle speed and the actuation speed of the working device can beretained in a relationship in which they are proportional to the enginerotational speed. Therefore, in the case where the work, such as therotary tillage work, is performed in which the vehicle speed and theactuation speed of the working device are desired to be retained in aconstant relationship, in the wide rotational speed region on thehigh-speed side, even though the engine rotational speed is changed dueto traveling load, working load or the like during work traveling, thevehicle speed and the actuation speed of the working device can beretained in the constant relationship.

Moreover, the decrease in the engine rotational speed from the secondset rotational speed caused by traveling load or working load can besuppressed, and a state in which the vehicle speed and the actuationspeed of the working device are retained in the constant relationshipcan be easily continued.

(6) The second set rotational speed is set to the rotational speed equalor close to the rated rotational speed of the engine. According to thisconfiguration, in the wide rotational speed region from the rotationalspeed equal or close to the idling rotational speed to the rotationalspeed equal or close to the rated rotational speed, the enginerotational speed as well as the change gear ratio of the continuouslyvariable transmission device is changed by the accelerator operation, arange of adjustment of the vehicle speed by the accelerator operationbecomes wider, and the acceleration and deceleration by the acceleratoroperation is facilitated.

(7) According to this configuration, by shifting the speed change stepof the stepped transmission device to the speed change step for working,the state can be switched to a first condition suitable for worktraveling, and by shifting the speed change step of the steppedtransmission device to the speed change step for moving, the state canbe switched to a second condition suitable for moving. Therefore, aneffort of shifting between the first condition and the second conditioncan be omitted.

(8) The change gear ratio of the continuously variable transmissiondevice corresponding to the engine rotational speed is corrected to anappropriate one on which a driver's intention is reflected using thesmallest change gear ratio set by the change gear ratio setting unit ascriterion.

(9) A phenomenon in which a driver senses a discomfort can be prevented,such as stopping traveling of the work vehicle caused by thecontinuously variable transmission device becoming neutral, due to thereduction in the engine rotational speed, even though the engine isoperated.

In addition, in the case where the engine rotational speed is reduced toa rotational speed equal to or below the first set rotational speed dueto the traveling load, the working load or the like, the traveling stateis retained without making the continuously variable transmission deviceneutral. Therefore, even in the low rotational speed region equal to orbelow the first set rotational speed, the engine can be imparted with aviscous property enough to be sensed by the driver. Therefore, thedriver is allowed to recognize overload on the engine in the lowrotational speed region, to thereby motivate the driver to take somemeasures, such as deceleration operation to reduce the load.

(10) The vehicle speed control unit changes the swash plate angle of thevariable capacity pump and the swash plate angle of the variablecapacity motor so that the vehicle speed read out from the output fromthe vehicle speed sensor matches the control target vehicle speedobtained based on the output from the engine sensor and the output fromthe change gear ratio setting unit.

(11) In a case where the engine rotational speed is increased from theidling rotational speed by the accelerator operation and the workvehicle is started, as the engine rotational speed becomes a lowerrotational speed close to the first set rotational speed, the changegear ratio of the hydrostatic continuously variable transmission devicebecomes the large change gear ratio on the low-speed side, and thus aload on the engine is reduced. Therefore, as compared with the casewhere the work vehicle is started in a state in which the change gearratio of the hydrostatic continuously variable transmission device isretained at a small change gear ratio on the high-speed side exhibitinga large load, a decrease in the engine rotational speed and generationof engine stall caused by overload upon starting can be effectivelysuppressed, and the starting of the work vehicle can be smoothlyperformed.

In addition, in a case where the engine rotational speed is reduced to arotational speed lower than the second set rotational speed duringtraveling due to the traveling load, working load or the like, as thedecrease amount from the second set rotational speed becomes larger, thechange gear ratio of the hydrostatic continuously variable transmissiondevice becomes a large change gear ratio on the low-speed side, and thusa load on the engine is reduced. Therefore, the engine can be impartedwith a viscous property, to thereby effectively suppress the generationof engine stall which may otherwise be caused by overload.

Then, when the engine rotational speed is increased to or above thesecond set rotational speed, regardless of fluctuation of the powertransmission efficiency in the hydrostatic continuously variabletransmission device caused by fluctuation in load, the change gear ratioof the hydrostatic continuously variable transmission device becomesconstant at the smallest change gear ratio, and therefore, regardless offluctuation of the engine rotational speed caused by the acceleratoroperation, traveling load or working load, a relationship in which thevehicle speed is proportional to the engine rotational speed isretained. Accordingly, in the case where the work, such as rotarytillage work, is performed in which the vehicle speed and the actuationspeed of the working device are desired to be retained in a relationshipin which they are proportional to the engine rotational speed, therelationship can be retained with high accuracy regardless of thefluctuation of the power transmission efficiency in the hydrostaticcontinuously variable transmission device, and a difference is unlikelyto occur between the vehicle speed and the appropriate speed for theactuation speed of the working device.

Furthermore, when the engine rotational speed is a rotational speedbetween the first set rotational speed and the second set rotationalspeed, the engine rotational speed as well as the change gear ratio ofthe hydrostatic continuously variable transmission device is changed bythe accelerator operation, a range of adjustment of the vehicle speed bythe accelerator operation becomes wider, and an amount of change in thevehicle speed relative to the accelerator operation amount becomeslarger, and acceleration and deceleration by the accelerator operationis facilitated.

(12) Even in the case of the configuration that has the steppedtransmission device, when work, such as rotary tillage work, isperformed in which the vehicle speed and the actuation speed of theworking device are desired to be retained in a relationship in whichthey are proportional to the engine rotational speed, the relationshipcan be retained with high accuracy, and a difference is unlikely tooccur between an appropriate speed for the actuation speed of theworking device and the vehicle speed.

(13) When the speed change step of the stepped transmission device ischanged, in accordance with this change, the computation result by thecomputing unit is changed to a large extent. Accordingly, the displayrange of the speed change step of the continuously variable transmissiondevice displayed on the speed change step display part for variablespeed change is automatically switched to the display rangecorresponding to the speed change step of the stepped transmissiondevice. In other words, the speed change step of the continuouslyvariable transmission device can be displayed with an appropriatedisplay range corresponding to each speed change step of the steppedtransmission device, and the vehicle speed setting corresponding to workcondition, including the shifting of the speed change step of thestepped transmission device, is facilitated.

(14) The speed change step of the continuously variable transmissiondevice and the speed change step of the stepped transmission device aredisplayed side by side. Accordingly, an association between the speedchange step of the continuously variable transmission device and thespeed change step of the stepped transmission device can be easilyunderstood.

(15) When the change in the set rotational speed of the engine by theengine rotational speed setting unit, the change in the change gearratio of the continuously variable transmission device by the changegear ratio setting unit or the like are performed, the set vehicle speeddisplayed on the set vehicle speed display part is changed in acontinuous manner in accordance with those changes. Therefore, in a casewhere work is performed with a working device in which a recommendedvehicle speed is set, such as sparging device, or in a case where workis performed at the desired engine rotational speed and vehicle speed,the vehicle speed setting can be simply and quickly performed regardlessof the state of the vehicle including traveling and stopping.

(16) The display on the set vehicle speed display part functions as aninterrupt display that works during the detection of the operation ofthe speed change operation tool, and during a set time after theoperation of the operation tool becomes undetectable. An amount ofinformation always displayed on the display device can be reduced, tothereby help easy understanding of the displayed information.Especially, even when the set rotational speed of the engine is set tothe rated rotational speed by the engine rotational speed setting unitand the speed change step of the continuously variable transmissiondevice displayed on the speed change step display part for variablespeed change becomes the same as the set vehicle speed displayed on theset vehicle speed display part, they can be easily distinguished.

(17) The vehicle speed is displayed on the vehicle speed display part.Even when the accelerator operation is maximized and the speed changestep of the continuously variable transmission device displayed on thespeed change step display part for variable speed change becomes thesame as the vehicle speed displayed on the vehicle speed display part,they can be easily distinguished.

(18) The speed change step of the variable capacity motor can be easilyrecognized. The difference in the feeling of traveling caused by thespeed change step of the variable capacity motor can be easilyrecognized.

Other Embodiments

<1> The work vehicle may be an agricultural work vehicle, e.g. ridingtype rice transplanter and combine; a riding type mower; or aconstruction work vehicle, e.g. wheel loader.

<2> The work vehicle may be: those having no auxiliary transmissiondevice (stepped transmission device) 10; those having a pair of theright and left continuously variable transmission devices 9 forindividually changing the speed of each of the right and left rearwheels 2; those having a main clutch; or those having a forward-reverseswitching mechanism specialized for switching between a forward movementand a reverse movement.

<3> The main transmission device (continuously variable transmissiondevice) 9 may be those having the variable capacity motor 32 which iscapable of performing three- or more step speed change, or ahydromechanical transmission device (HMT) into which a planetary gear isincorporated.

<4> The swash plate angle control unit D may be, for example, configuredto perform the swash plate feedback control in which the actuation ofthe pump swash plate operation unit A is controlled in such a mannerthat the output of the swash plate angle sensor for detecting theoperation angle of the pump swash plate 31B matches the control targetoperation angle which is set based on the operation position of the mainshift lever 20 (the operation angle of the swash plate angle sensorfalls on the dead band of the control target operation angle).

<5> With respect to the configurations of the pump swash plate operationunit A and the motor swash plate operation unit B, various modificationsare possible, and for example, an electric actuator, such as electricmotor and electromagnetic cylinder, may be adopted.

<6> The auxiliary transmission device 10 may be capable of performingtwo-step speed change including the low-speed step for working and thehigh-speed step for moving.

<7> The lever sensor (speed change step detector) 60 for stepped speedchange may be those configured to detect an operation position of ashift member for switching speed change step provided in the auxiliarytransmission device 10.

<8> The speed change operation tool 20 may be those having a main shiftpedal and the like, or alternatively, those having both the main shiftlever and the main shift pedal.

<9> The speed change operation tool 20 may not have the medium-speed setregion (hysteresis), and may be configured to be operated to thelow-speed set region and the high-speed set region.

<10> The change gear ratio determining unit 27Ba may be those configuredto detect the operation angle of the pump swash plate 31B based on anamount of electrical continuity to the variable speed valve 38 forforward traveling or the variable speed valve 39 for reverse traveling,and to, based on whether or not the detected operation angle of the pumpswash plate 31B reaches the operation limitation angle, determinewhether or not a change gear ratio of the main transmission device 9which is set based on the output of the lever sensor (operation positiondetector) 59 is a change gear ratio that can be attained by the swashplate angle of the variable capacity motor 32 on the low-speed side.Alternatively, the change gear ratio determining unit 27Ba may be thoseprovided with the swash plate angle sensor for detecting the operationangle of the pump swash plate 31B, and configured to, based on whetheror not the operation angle of the pump swash plate 31B obtained from anoutput of the swash plate angle sensor reaches the operation limitationangle, determine whether or not the change gear ratio of the maintransmission device 9 which is set based on the output of the leversensor 59 is a change gear ratio that can be attained by the swash plateangle of the variable capacity motor 32 on the low-speed side.

<11> It may be configured that, when the swash plate angle control unitD or the first switching control unit 27Bc reads out the operationposition of the speed change operation tool 20 based on the output ofthe lever sensor 59 and switches the speed change step of the variablecapacity motor 32 in accordance with the operation region where thespeed change operation tool 20 is located, a speed change operation ofthe pump swash plate 31B, with the operation amount which is set so asto compensate (offset) the amount of change in the change gear ratio ofthe main transmission device 9 caused along with the switching, is notperformed, and only when the swash plate angle control unit D or thesecond switching control unit 27Bd switches the speed change step of thevariable capacity motor 32 based on the determination result by thechange gear ratio determining unit 27Ba configured to determine thechange gear ratio of the main transmission device 9 which is set basedon the output of the lever sensor 59, the speed change operation of thepump swash plate 31B, with the operation amount which is set so as tocompensate (offset) the amount of change in the change gear ratio of themain transmission device 9 caused along with the switching, isperformed.

<12> With respect to the engine drop amount (set amount) which is usedas criterion for determining to perform the switching of the speedchange step of the variable capacity motor 32 from the high-speed stepto the low-speed step by the swash plate angle control unit D or thethird switching control unit 27Be, various modifications are possibledepending on the performance of the engine 3 and types of the workperformed by the work vehicle.

<13> The swash plate angle control unit D or the third switching controlunit 27Be may be configured in such a manner that, when it switches thespeed change step of the variable capacity motor 32 from the high-speedstep to the low-speed step based on the engine drop amount and it isdetected that the engine rotational speed is recovered to the setrotational speed based on the output of the decrease detection unit27Bb, the swash plate angle control unit D or the third switchingcontrol unit 27Be switches the speed change step of the variablecapacity motor 32 from the low-speed step to the high-speed step.

<14> The vehicle speed control unit 27B may be, for example, configuredto perform the swash plate feedback control in which the actuation ofthe pump swash plate operation unit A is controlled in such a mannerthat the output from the swash plate angle sensor for detecting theoperation angle of the pump swash plate 31B matches the control targetoperation angle which is set based on the operation position of the mainshift lever 20 (the operation angle of the swash plate angle sensorfalls on the dead band of the control target operation angle).

<15> The vehicle speed control unit 27B may be configured to correct thechange gear ratio of the main transmission device 9 corresponding to theengine rotational speed in the medium rotational speed region NM and thehigh rotational speed region NH, based on the output from the changegear ratio setting unit.

<16> The first set rotational speed in the vehicle speed setting datamay be set to a rotational speed close to the idling rotational speed.In addition, the second set rotational speed in the first vehicle speedsetting data may be set to a rotational speed close to the rotationalspeed at which the engine 3 exerts the maximum torque. Further, thefirst change gear ratio and the second set rotational speed in thevehicle speed setting data may be configured to be changeable inaccordance with type of the work vehicle and type of the working deviceto be installed.

<17> An operation tool may be provided, such as switch for switching thevehicle speed setting state between the first condition in which thefirst vehicle speed setting data is adopted and the second condition inwhich the second vehicle speed setting data is adopted.

<18> Both the vehicle speed setting data specialized for low-speedworking and the vehicle speed setting data specialized for high-speedworking may be provided.

<19> The vehicle speed sensor (vehicle speed detector) 63 may be thoseconfigured to detect the output rotational speed of the auxiliarytransmission device 10, or those provided on both right and left rearwheel shafts so as to detect an output rotational speed of thedifferential device 13 for rear wheel. Then, in accordance with theconfiguration of the vehicle speed sensor 63, a manner the vehicle speedcontrol unit 27B obtains the control target vehicle speed for the outputfrom the vehicle speed sensor 63 is also to be modified.

<20> The speed change step of the variable capacity motor 32 may bedirectly detected by a swash plate angle sensor for detecting theoperation angle of the motor swash plate 32B, or indirectly detected(estimated) from an operation signal to the motor swash plate 32B.

The speed change system for the work vehicle according to the presentinvention is applicable to the work vehicle, such as tractor, ridingtype mower, riding type rice transplanter, combine, and wheel loader,which has the hydrostatic transmission device in which the swash plateangle of the variable capacity pump is non-stepwise changeable and theswash plate angle of the variable capacity motor is stepwise changeable.

What is claimed is:
 1. A work vehicle comprising: right and left rearwheels; a continuously variable transmission device operable to changespeed of traveling power of the vehicle steplessly; right and left brakepedals, each of the brake pedals being operable to be pressed across abraking canceling region, a braking speed change region definedcontinuous with the braking canceling region, and a braking stop regiondefined continuous with the braking change speed region; a right brakebraking the right rear wheel with a braking force in accordance with apressing operation amount the right brake pedal; a left brake brakingthe left rear wheel with a braking force in accordance with a pressingoperation amount the left brake pedal; a pedal sensor for detecting adual pressing operation position of the right and left brake pedals; anda vehicle speed control unit, wherein when the dual pressing operationposition of the right and left brake pedals is within the braking changespeed region, the vehicle speed control unit controls the continuouslyvariable transmission device toward a decelerating side thereof, inaccordance with a dual pressing operation amount of the right and leftbrake pedals within the braking change speed region.
 2. A work vehiclecomprising: right and left rear wheels; a continuously variabletransmission device operable to change speed of traveling power of thevehicle steplessly; right and left brake pedals, each of the brakepedals being operable to be pressed across a braking canceling region, abraking speed change region defined continuous with the brakingcanceling region, and a braking stop region defined continuous with thebraking change speed region; a right brake braking the right rear wheelwith a braking force in accordance with a pressing operation amount theright brake pedal; a left brake braking the left rear wheel with abraking force in accordance with a pressing operation amount the leftbrake pedal; a vehicle speed sensor for detecting a speed of thevehicle; a pedal sensor for detecting a dual pressing operation positionof the right and left brake pedals; and a vehicle speed control unit forchanging a change speed ratio of the continuously variable transmissiondevice, wherein when the dual pressing operation position of the rightand left brake pedals is within the braking canceling region, thevehicle speed control unit changes the change speed ratio of thecontinuously variable transmission device in such a manner that thespeed of the vehicle detected by the vehicle speed sensor matches acontrol target vehicle speed.
 3. A work vehicle according to claim 2,wherein when the dual pressing operation position of the right and leftbrake pedals is within the braking change speed region, the vehiclespeed control unit changes the change speed ratio of the continuouslyvariable transmission device in such a manner that, as the dual pressingoperation position of the right and left brake pedals becomes large, thespeed of the vehicle detected by the vehicle speed sensor approacheszero.
 4. A work vehicle according to claim 3, wherein when the dualpressing operation position of the right and left brake pedals is withinthe braking stop region, the vehicle speed control unit neutralizes thecontinuously variable transmission device in such a manner that thespeed of the vehicle detected by the vehicle speed sensor zeroesregardless of the control target vehicle speed.